Add rubocop to lint in default rake task

Disabling the other cops because they are quite opinionated and noisy. The
lint checks are the one we should really care about though, because they
highlight code smells.
Fix assignment in condition lint warnings

This is confusing for the small amount of characters it saves. Most of these
can refer to the original variable instead.

The exception is `input_pool_service_port[:health_check]` which referred to
the new var twice and the old var once within the condition. I've replaced
the ternary condition with `.fetch(key, default).to_s` to do the same thing
and preserve line length.
Remove unused protocol arg in LoadBalancerService

This argument is accepted and passed but not ever used. So remove it to fix
lint warning.
Remove unused var from publish_gem task

This variable isn't used, so we don't need to accept it. Fixes lint warning.
Fix indent in configuration_differ_shared_examples

The `end` was off by one character. Fixes lint warning.
Remove rescue and extra requires from binstub

We're not sure why these are here and lint complains about silencing the
`LoadError` exception. So remove them, but carry the methadone require
through.
Merge pull request #47 from alphagov/add_rubocop_lint

Add rubocop to lint in default rake task + fix warnings