Added utf-8 encoding comment

Without this, RubyMine 5 gets an error when loading it, so it doesn't:

```
2013-03-02 16:12:29,588 [   7852]  ERROR - by.ruby.lang.RubySupportLoader - Error loading script /Users/ed/Development/rubymine-extensions/plugins/rspec_context/rspec_context.rb 
org.jruby.exceptions.RaiseException: (SyntaxError) /Users/ed/Development/rubymine-extensions/plugins/rspec_context/rspec_context.rb:276: invalid multibyte char (US-ASCII)
```