Minor grep command fix

I was seeing an issue where, when boxen was checking to see if a key was already set it was calling grep with the search term unquoted, causing grep to try and grep inside a file by default in OS X 10.10 if there are spaces in the term.

Ie was:
    grep enable_capslock 80
should have been:
    grep 'enable_capslock 80'

This patch should be all fine unless Seil starts using single quotes in it's commands.