Use the sexp_processor gem instead of the code_analyzer gem

The sexp_processor gem is a dependency of the code_analyzer gem. Most of
the functionality that RubyCritic depends on is part of the
sexp_processor gem, and not the code_analyzer gem. For that reason, I'd
rather go down one level and depend on less code.
Test Analyser::Stats
Stop testing Rubinius given this gem no longer supports it :(

Unfortunately, to count the number of methods, I'm using Ripper [1], a
Ruby script parser. Currently, Rubinius does not implement Ripper [2] so
I should probably:

1. Either use this feature only when the user is on some Ruby other than
Rubinius. This would allow using RubyCritic on Rubinius, but it would be
lacking a feature.

2. Or even better, switch from Ripper to Parser [3] and support all
features in all Rubies.

[1]: http://ruby-doc.org/stdlib-2.1.2/libdoc/ripper/rdoc/Ripper.html

[2]: https://github.com/rubinius/rubinius/issues/2377

[3]: https://github.com/whitequark/parser/issues/21