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

Enable Style/AndOr cop only for lib/puppet/pops directory
My reasoning was that I was making explicit the implicit precedence rules of `and` and `&&` and hence making it easier to visually understand which expressions were involved. I had seen in ruby style guide that parenthesizing completely as in if (a ==b && c == d) is [discouraged](https://github.com/bbatsov/ruby-style-guide#no-parens-if), and it seems that parenthesis are [encouraged](http://programmers.stackexchange.com/questions/201175/should-i-use-parentheses-in-logical-statements-even-where-not-necessary) in places such as this.