When PUPPET_VERSION="~> x.x.x" bundler breaks

If I want to specify the puppet version with "~> x.x.x" bundler will
complain saying that requirements are not met.

With this change you can specify the puppet version as:
PUPPET_VERSION="x.x.x"
PUPPET_VERSION="~> x.x.x"
PUPPET_VERSION="<= x.x.x"
etc.