Install aspell and libaspell-dev for Travis tests

Should fix failing Travis tests on the `spelling_suggestions` branch.

This will execute before the repo is cloned. `update` is required because
APT's cache may be pointing at stale mirror locations. `-qq` makes APT super
quiet, so that it doesn't clog up your normal test output. It should however
still give a meaningful message if it fails during the `apt-get`.
Include aspell-en for Travis tests also

Provides English dictionaries. The main `aspell` package recommends this,
rather than depends on it. Travis tests currently pass without it - but we
think it may be required and it maintains parity with our Puppet manifests.
Merge pull request #106 from alphagov/spelling_suggestions-travis_deps

Install aspell and libaspell-dev for Travis tests