Add a version specifier for rspec in Gemfile

Specify rspec 2.14.1 because rspec 3.x has some backwards incompatible changes which break the current spec.
Fix case statement handling in max-nested-depth rule

The rule now works for case statements without a testable (e.g. case;when a;b;end).
That use of case statements doesn't really makes a lot of sense, but it's valid ruby code,
so it's unacceptable for skeptic to just crash on it.