Skip 'import' statements when parser = future

As mentioned in https://github.com/rodjek/rspec-puppet/issues/196#issuecomment-54631457, this looks to fix rspec-puppet when the future parser is used. I tested a module using Puppet 3.2, 3.5 and 3.7, and since the default is to leave behavior default when `parser != future` this shouldn't affect any existing tests when using the default parser.

If spec tests are required I'll need some help; I'm not sure how best to test the `load_catalogue` method that the code has updated.

Fixes #196