Add package spell checking

[jeroen]

A quick way to find spelling errors in a package title, description and manual pages. More rigorous than CRAN's checks... you might find a few spelling errors in devtools :)

[jimhester]

libhunspell-dev is in the whitelist for ubuntu-precise, so this should be using the apt addon rather than using sudo (https://github.com/travis-ci/apt-package-whitelist/blob/master/ubuntu-precise#L5584)

addons:
  apt:
    packages:
      - libhunspell-dev

[jeroen]

Thanks, fixed.

[hadley]

Thanks! Maybe this should be added to release() as well? (i.e. "Have you used spell_check() to check for spelling errrors?")