(PUP-3082) When PUPPET_VERSION="~> x.x.x" bundler breaks

Without this change, bundler breaks if PUPPET_VERSION is specified with a curvy arrow syntax for example export PUPPET_VERSION="~> 3.6.0".

Without this change the puppet gem declaration after processing would look like 'gem "puppet", "= ~> 3.6.0"' that isn't valid syntax.