ActiveRecord & ruby optimizations

[AR & Ruby Optimizations - Bounty 435](https://assembly.com/coderwall/bounties/435)
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- select(:foo).map(:foo) -> pluck(:foo)
- select(:foo).collect(:foo) -> pluck(:foo)
- map { |foo| foo.bar } -> map(&:bar)
- map { … }.flatten -> flat_map { … }
- collect { … }.flatten -> flat_map { … }
- gsub() -> sub()
- gsub() -> tr()