Improve code style
Improve code style

* Prefer $() to ``
* Quote expansions

adrianheine/gitsh,adrianheine/gitsh,jakirkham/gitsh,thoughtbot/gitsh,jakirkham/gitsh,andybalaam/gitsh,thoughtbot/gitsh,andybalaam/gitsh,thoughtbot/gitsh,jakirkham/gitsh,andybalaam/gitsh