Stop Parser warning us about Ruby 2.1.4 syntax
Stop Parser warning us about Ruby 2.1.4 syntax

Rubocop requires Parser, which warns about mythical Ruby versions when
running any scripts in development:

```
warning: parser/current is loading parser/ruby21, which recognizes
warning: 2.1.5-compliant syntax, but you are running 2.1.4.
```

Making the Rubocop Gem load only when explicitly asked means we don't
see these warnings anymore.

robinwhittleton/whitehall,ggoral/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,ggoral/whitehall,hotvulcan/whitehall,alphagov/whitehall,askl56/whitehall,alphagov/whitehall,askl56/whitehall,hotvulcan/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,askl56/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall