Add RubyParser helper class

Add helper class that encapsulates the `parser` and `astrolabe` gems so
that all linters need to do is call `parse_ruby` to get an AST.

Change-Id: I3ae6d7899766ff791ca8107593e115aae20ae97e
Reviewed-on: http://gerrit.causes.com/42570
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Add ClassAttributeWithStaticValue linter

Add linter which checks for the declaration of class attributes via hash
attributes and assigning them static values.

Change-Id: I1d7b919795f276642577e5b1aad43350b79d1288
Reviewed-on: http://gerrit.causes.com/42571
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>