Get rid of noisy gems and profit

* Bullet fills the logs with noise regarding potential n+1 queries but
is often wrong and basically ignored
* rack-mini-profile is very obstructive (blocks the "import" link),
slows page loading down considerably and is basically ignored
* rails-dev-boost no longer provides much of a boost as rails
optimises code reloading pretty well itself.
Merge pull request #1320 from alphagov/ditch-ignored-gems

Get rid of noisy gems and profit