Clarify `path` command line option in the README

Make a little more obvious the difference between using relative and
absolute paths.
Require Reek 1.3.8

This version of Reek has a looser ruby2ruby dependency, thus allowing
the use of ruby2ruby 2.1.1. This version fixes a bug that prevented the
use of Reek alongside Flay [1].

[1]: https://github.com/seattlerb/ruby_parser/issues/154
Update Stats test to also check if class methods are detected
Use Parser instead of Ripper and Sexp_processor

This means we can support Rubinius again!

(And this was easy after all!)
Return a Rubycritic::AST::EmptyNode when an analysed file is empty

Otherwise Parser just returns nil, which blows up later on.

I must say, I don't like this code organization very much. I'm monkey
patching Parser::AST::Node and creating a new class,
Rubycritic::AST::EmptyNode, and placing them both in
`rubycritic/analysers/ast_node.rb`. I think these two classes should be
in the same file, but I'm not sure **where** that file should be placed.
For now, this will do.
Update Stats test to check that it can deal with empty Ruby files
Start testing Rubinius again :D
Specify Ruby 2.1 in Travis config

Since Ruby 2.1.0, the MRI versioning policy changed to follow Semantic
Versioning [1].

Thus, it is better to use '2.1' instead of '2.1.x' in Travis config
since this way the latest patch release will be selected automatically.

[1]:
https://www.ruby-lang.org/en/news/2013/12/21/ruby-version-policy-changes-with-2-1-0/