fix rspec backtrace on launch

Not sure if this is the right way to fix it, but without this change `rspec` failed to execute (with or without arguments):
```
/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:51:in `require': cannot load such file -- pio (LoadError)
	from /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /tmp/trema/spec/spec_helper.rb:27:in `<top (required)>'
	from /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /tmp/trema/spec/trema/app_spec.rb:19:in `<top (required)>'
	from /usr/local/share/gems/gems/rspec-core-2.14.5/lib/rspec/core/configuration.rb:896:in `load'
	from /usr/local/share/gems/gems/rspec-core-2.14.5/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
	from /usr/local/share/gems/gems/rspec-core-2.14.5/lib/rspec/core/configuration.rb:896:in `each'
	from /usr/local/share/gems/gems/rspec-core-2.14.5/lib/rspec/core/configuration.rb:896:in `load_spec_files'
	from /usr/local/share/gems/gems/rspec-core-2.14.5/lib/rspec/core/command_line.rb:22:in `run'
	from /usr/local/share/gems/gems/rspec-core-2.14.5/lib/rspec/core/runner.rb:80:in `run'
	from /usr/local/share/gems/gems/rspec-core-2.14.5/lib/rspec/core/runner.rb:17:in `block in autorun'
```