Bump the min rspec development dependency to 2.13

The punchblock spec suite implicitly requires rspec 2.13 or higher.  Anything less, and you get unexpected errors.  This just formalizes the updated dependency requirement.

On rspec < 2.12, you'd get this before any tests ran at all:

$ cd punchblock
$ bundle exec rspec spec

```
/Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:334:in `mock_with': RSpec::Core::MockFrameworkAdapter must respond to `configuration` so that mock_with can yield it. (RuntimeError)
  from /Users/sgeorge/Code/adhearsion/adhearsion-punchblock/spec/spec_helper.rb:19:in `block in <top (required)>'
  from /Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core.rb:92:in `configure'
  from /Users/sgeorge/Code/adhearsion/adhearsion-punchblock/spec/spec_helper.rb:15:in `<top (required)>'
  from /Users/sgeorge/Code/adhearsion/adhearsion-punchblock/spec/punchblock/client/component_registry_spec.rb:3:in `require'
  from /Users/sgeorge/Code/adhearsion/adhearsion-punchblock/spec/punchblock/client/component_registry_spec.rb:3:in `<top (required)>'
  from /Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:780:in `load'
  from /Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:780:in `block in load_spec_files'
  from /Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:780:in `map'
  from /Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:780:in `load_spec_files'
  from /Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core/command_line.rb:22:in `run'
  from /Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core/runner.rb:69:in `run'
  from /Users/sgeorge/.rvm/gems/ruby-1.9.3-p545/gems/rspec-core-2.11.1/lib/rspec/core/runner.rb:8:in `block in autorun'```

Even more confusing on rspec == 2.12.0... you'd get several test failures.

2.13.0 seems to be the minimum version for tests to both run and pass.