resolving gemfile & gemspec conflict

conflicts happens because gemspec expects a particular
version of 'fog' gem, whereas Gemfile tries
to use the latest master version

resolved this conflict by changing gemspec to accept a version
greater than to equal to a particula version.
this allows user to override version in Gemfile.
Merge pull request #81 from alphagov/fixing_gem_spec

resolving gemfile & gemspec conflict