Isolate future_parser tests with rspec context

This ensures that `PuppetSyntax.future_parser` doesn't persist as true after
the test which previously set it in an `it{}` block.

I've also changed the test for the default behaviour of `false` to check the
current value instead of set it.
Test for future_parser on Puppet <= 3.2

This extends our coverage for the other side of the `if` condition on
`Puppet.version` and specifies what will happen if someone tries to use this
option on an older version of Puppet.
Merge pull request #10 from gds-operations/more_future_parser_tests

Test for future_parser on Puppet <= 3.2