Be less restrictive on the rubocop version required.

`~> 0.41.0` is too restrictive. Will only work with `0.41.x` and will basically require this to be updated on each new rubocop release. `~> 0.41` grants us dependency sanity on `0.x.y`.

rubocop-migrations.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
-  spec.add_runtime_dependency 'rubocop', '~> 0.41.0'
+  spec.add_runtime_dependency 'rubocop', '~> 0.41'
 
   spec.add_development_dependency 'bundler', '~> 1.11'
   spec.add_development_dependency 'rake', '~> 10.0'