Use ~> 0.22.0 for rubocop dependency

With Rubocop 0.23.0, the specs were blowing up with:

> haml-lint/lib/haml_lint/linter/ruby_script.rb:75:in
> `<module:HamlLint>': uninitialized constant HamlLint::Rubocop
> (NameError)

To fix this, I am setting the version dependency at the last known
version that works.

Change-Id: I425fb923dbd098ad35888e66594bda9ad2738d3f
Reviewed-on: http://gerrit.causes.com/38694
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Henric Trotzig <henric.trotzig@causes.com>
Rename Rubocop to RuboCop, adjust dependency version

In v0.23.0 of RuboCop, the Rubocop module was renamed to RuboCop[1]. So
that we can continue using newer versions of this gem as they are
released, I am updating our naming and bumping the dependency version.

[1]: https://github.com/bbatsov/rubocop/commit/ff167d8f202baf7a68955db0aaf0dc29afb7e7ee

Change-Id: Iba542493f28a192dd46aed6364e3c1dca43c81a0
Reviewed-on: http://gerrit.causes.com/38696
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@brigade.com>