[Fix rubocop#79] Rubocop::Cop::Rails constant removal

Before, it attempted to remove the constant whether Rubocop::Cop::Rails was
defined or if ::Rails was defined. Turning off the inherit flag while checking
const_defined? remedies this.

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Bug fixes
 
 * [#43](https://github.com/rubocop-hq/rubocop-rails/issues/43): Remove `change_column_null` method from `BulkChangeTable` cop offenses. ([@anthony-robin][])
+* [#79](https://github.com/rubocop-hq/rubocop-rails/issues/79): Fix `RuboCop::Cop::Rails not defined (NameError)`. ([@rmm5t][])
 
 ### Changes
 
@@ -27,3 +28,4 @@
 [@andyw8]: https://github.com/andyw8
 [@buehmann]: https://github.com/buehmann
 [@anthony-robin]: https://github.com/anthony-robin
+[@rmm5t]: https://github.com/rmm5t

lib/rubocop/cop/rails_cops.rb

@@ -4,7 +4,7 @@ module RuboCop
   # RuboCop included the Rails cops directly before version 1.0.0.
   # We can remove them to avoid warnings about redefining constants.
   module Cop
-    remove_const('Rails') if const_defined?('Rails')
+    remove_const('Rails') if const_defined?('Rails', false)
   end
 end