rake tasks are broken with latest rubygem versions (v. 2 or later)

Method `ForkedCucumberRunner#gem_available?` caused an error in rake with
the message _'undefined method `available?` for Gem:Module'_.
This occurred when rubygems could not find a gem, then the method falled back to a legacy call to the `Gem::Module#available?` that doesn't exist anymore in rubygems 2 and
above. The fix consists in avoiding to call the obsolete method when Rubygems has version 2 or above.