Don't require support/external_request from rake tasks

If we require this we're requiring everything that
support/external_request requires for production.

Gemfile

@@ -159,9 +159,6 @@ gem 'bootstrap-switch-rails', '~> 3.0.0'
 # for parsing OEmbed data
 gem 'ruby-oembed'
 
-# for mocking external requests
-gem 'webmock'
-
 # Use guard and spring for testing in development
 group :development do
   # rspec Guard rules
@@ -196,6 +193,8 @@ group :test do
   # Extracted from RSpec 3 stub_model and mock_model
   gem 'rspec-activemodel-mocks'
   gem 'timecop'
+  # for mocking external requests
+  gem 'webmock'
 end
 
 group :development, :test do