Fix the Rubinius build

The build is broken in Rubinius. I'm not sure why. I tried to fix it with this PR, but that failed. 

https://travis-ci.org/orgsync/active_interaction/jobs/43514055#L332

```
Coercion error: nil.to_int => Fixnum failed
kernel/common/type.rb:36:in `coerce_to_failed'
kernel/common/type.rb:152:in `coerce_to_collection_length'
kernel/common/array.rb:57:in `initialize'
kernel/common/array.rb:1664:in `to_a'
/home/travis/.rvm/gems/rbx-2.4.1/gems/ast-2.0.0/lib/ast/node.rb:61:in `initialize'
/home/travis/.rvm/gems/rbx-2.4.1/gems/astrolabe-1.3.0/lib/astrolabe/node.rb:27:in `initialize'
/home/travis/.rvm/gems/rbx-2.4.1/gems/astrolabe-1.3.0/lib/astrolabe/builder.rb:25:in `n'
/home/travis/.rvm/gems/rbx-2.4.1/gems/parser-2.2.0.pre.8/lib/parser/builders/default.rb:182:in `array'
ruby21.y:981:in `_reduce_291'
:in `call'
kernel/bootstrap/proc.rb:20:in `call'
kernel/common/throw_catch.rb:30:in `catch'
kernel/common/throw_catch.rb:7:in `register'
kernel/common/throw_catch.rb:29:in `catch'
cparse.c:in `_racc_do_parse_c'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/racc-1.4.12/lib/racc/parser.rb:271:in `do_parse'
/home/travis/.rvm/gems/rbx-2.4.1/gems/parser-2.2.0.pre.8/lib/parser/base.rb:175:in `parse'
/home/travis/.rvm/gems/rbx-2.4.1/gems/parser-2.2.0.pre.8/lib/parser/base.rb:193:in `parse_with_comments'
/home/travis/.rvm/gems/rbx-2.4.1/gems/parser-2.2.0.pre.8/lib/parser/base.rb:214:in `tokenize'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/processed_source.rb:67:in `parse'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/processed_source.rb:27:in `initialize'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/processed_source.rb:15:in `from_file'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/runner.rb:54:in `process_file'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/runner.rb:27:in `run'
kernel/bootstrap/array.rb:76:in `each'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/runner.rb:25:in `run'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/cli.rb:24:in `run'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/rake_task.rb:50:in `run_cli'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/rake_task.rb:38:in `run_main_task'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/rake_task.rb:30:in `initialize'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/file_utils_ext.rb:58:in `verbose'
/home/travis/.rvm/gems/rbx-2.4.1/gems/rubocop-0.27.1/lib/rubocop/rake_task.rb:26:in `initialize'
kernel/bootstrap/proc.rb:20:in `call'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:240:in `execute'
kernel/bootstrap/array.rb:76:in `each'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:235:in `execute'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:179:in `invoke_with_call_chain'
/home/travis/.rvm/rubies/rbx-2.4.1/gems/gems/rubysl-monitor-2.0.0/lib/rubysl/monitor/monitor.rb:211:in `synchronize (mon_synchronize)'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:172:in `invoke_with_call_chain'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:201:in `invoke_prerequisites'
kernel/bootstrap/array.rb:76:in `each'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:199:in `invoke_prerequisites'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:178:in `invoke_with_call_chain'
/home/travis/.rvm/rubies/rbx-2.4.1/gems/gems/rubysl-monitor-2.0.0/lib/rubysl/monitor/monitor.rb:211:in `synchronize (mon_synchronize)'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:172:in `invoke_with_call_chain'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/task.rb:165:in `invoke'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/application.rb:150:in `invoke_task'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/application.rb:106:in `top_level'
kernel/bootstrap/array.rb:76:in `each'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/application.rb:106:in `top_level'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/application.rb:115:in `run_with_threads'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/application.rb:100:in `top_level'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/application.rb:78:in `run'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/application.rb:176:in `standard_exception_handling'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/lib/rake/application.rb:75:in `run'
/home/travis/.rvm/gems/rbx-2.4.1@global/gems/rake-10.4.2/bin/rake:33:in `__script__'
kernel/common/kernel.rb:497:in `load'
/home/travis/.rvm/gems/rbx-2.4.1/bin/rake:23:in `__script__'
kernel/common/block_environment.rb:53:in `call_on_instance'
kernel/common/eval.rb:176:in `eval'
/home/travis/.rvm/gems/rbx-2.4.1/bin/ruby_executable_hooks:15:in `__script__'
kernel/delta/code_loader.rb:66:in `load_script'
kernel/delta/code_loader.rb:152:in `load_script'
kernel/loader.rb:645:in `script'
kernel/loader.rb:799:in `main'
```