Do not githubify sign-off messages

Don't successfully `githubify` strings prepended with an `@` symbol if
what's matched actually resembles an email address. @mentions should
only be githubified if there is a word boundary before it.

This fixes travis-ci/travis-ci#1591

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #231 from davidcelis/master

Do not githubify sign-off messages