Specify Rails version correctly.

gemfiles/Gemfile.rails4.1

@@ -6,7 +6,7 @@ group :development, :test do
   gem 'bson_ext', platforms: :ruby
   gem 'geoip'
   gem 'rubyzip'
-  gem 'rails', '>= 4.1'
+  gem 'rails', '~> 4.1.13'
   gem 'test-unit' # needed for Ruby >=2.2.0
 
   gem 'byebug', platforms: :mri
@@ -35,4 +35,4 @@ group :test do
     gem 'jdbc-sqlite3'
     gem 'activerecord-jdbcpostgresql-adapter'
   end
-end
+end
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