Fix Gemfile for travis

inside .travis.yml we worked with PUPPET_VERSION like ~> 3.0.0 which
gives us the latest version inside the same major release. This did not
work together with the Gemfile that is created with puppet module
generate, because it expects fixed versions.

Modify the Gemfile to allow the old behaviour again.

Gemfile

 source 'https://rubygems.org'
 
-puppetversion = ENV.key?('PUPPET_VERSION') ? "= #{ENV['PUPPET_VERSION']}" : ['>= 2.7']
-facterversion = ENV.key?('FACTER_VERSION') ? "= #{ENV['FACTER_VERSION']}" : ['>= 1.6']
+puppetversion = ENV.key?('PUPPET_VERSION') ? ENV['PUPPET_VERSION'] : ['>= 2.7']
+facterversion = ENV.key?('FACTER_VERSION') ? ENV['FACTER_VERSION'] : ['>= 1.6']
 
 gem 'puppet', puppetversion
 gem 'puppetlabs_spec_helper', '>= 0.1.0'