(PUP-2950) rubocop - Style/AndOR enabled for pops

Enable Style/AndOr cop only for lib/puppet/pops directory
There were a few reasons I went for this, 
* I felt that adding parenthesis was harmless, while improving clarity of the expression (given that we already had [problems with precedence](https://github.com/puppetlabs/puppet/pull/2722) previously.) 
* While doing the conversion, it was a bit more easier to convert, and verify visually that indeed the changes were correct, especially since we are not fixing the assignment in condition with this pull request (The bracketed form is correct with both = and ==, but incorrect with =).