drop mock dependency

mock is not used anywhere in the module, let's drop it.
This patch will help to hit this bug:
https://launchpad.net/bugs/1492636

Change-Id: I9513090167febbb5f826a3f2a9d628842086adc0

Gemfile

@@ -17,7 +17,6 @@ group :development, :test do
   gem 'puppet-lint-numericvariable'
 
   gem 'json'
-  gem 'webmock'
   gem 'minitest', :require => false
   gem 'test', :require => false
   gem 'test-unit', :require => false