Only add dependency on rubocop if we're actually going to use it

We only attempt to run rubocop from Ruby v2.0.0 onwards, so there's no
point in including it for earlier versions.

mocha.gemspec

@@ -46,7 +46,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
     # No test libraries in standard library
     s.add_development_dependency('minitest')
   end
-  if RUBY_VERSION >= '1.9.2'
+  if RUBY_VERSION >= '2.2.0'
     s.add_development_dependency('rubocop', '<= 0.58.2')
   end
   if ENV['MOCHA_GENERATE_DOCS']