Fix rubocop version constraint for Ruby > v2.2.0

For some reason the `jaro_winkler` version constraint added in this
commit [2] was causing bundler to downgrade the `rubocop` version from
v0.58.2 to v0.56.0. Unfortunately this resulted in some `rubocop`
violations when running `bundle exec rake lint` which prevented me from
releasing. Adding this lower bound to the `rubocop` version seems to
solve the problem.

[1]: 5c7d14c

Gemfile

@@ -23,7 +23,7 @@ if RUBY_VERSION >= '2.2.0'
 end
 if RUBY_VERSION >= '2.2.0'
   gem 'jaro_winkler', '>= 1.5.5'
-  gem 'rubocop', '<= 0.58.2'
+  gem 'rubocop', '> 0.56', '<= 0.58.2'
 end
 if ENV['MOCHA_GENERATE_DOCS']
   gem 'redcarpet'