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`.

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I noticed your tests were failing. Based on what we talked about yesterday. HTH.