Remove pin of fast_gettext

This is no longer required because ruby >= 2.1 is already available[1].

[1] b93ecf8bff

Change-Id: I91854630489c92307e8e84f8695ccd4a57362af9
(cherry picked from commit 3860bfc936)
(cherry picked from commit 7620fa93ca)
(cherry picked from commit 05c6ba8201)

Gemfile

@@ -11,7 +11,6 @@ group :development, :test, :system_tests do
         :ref => 'stable/wallaby',
         :require => 'false'
   end
-  gem 'fast_gettext', ['< 1.2.0']
 end
 
 if facterversion = ENV['FACTER_GEM_VERSION']