Fix example grammar

This pull request fixes the RSpec example grammar.  

Simply put, it turns this:

![before](https://cloud.githubusercontent.com/assets/5688/4264734/be43166c-3c31-11e4-81fd-ba58515e3017.png)

into this:

![after](https://cloud.githubusercontent.com/assets/5688/4264736/c46e7a04-3c31-11e4-8054-19a1e62d090e.png)

The problem with the current grammar is two-fold:

1. It hasn't kept up with the times as new example keywords have been introduced.
1. It tries to be too clever by capturing Ruby constructs (e.g. strings and single-line block literals).

This PR fixes the above problems by:

1. Matching all the RSpec 3.1 example keywords into one `keyword.other.exxample.rspec` named group.
1. Ignoring the surrounding Ruby constructs, leaving them to be parsed by the `language-ruby` grammar.

In short, it's a back-to-the-basics kind of PR.