Check whether the string contains, rather than is

With newer git's (In my case, 1.9.0 from homebrew), the message is a bit different:

`no changes added to commit (use "git add" and/or "git commit -a")`
instead of plain
`no changes added to commit`

To fix this, and other potential variations on the string, this just checks whether the string contains the phrase instead of matching exactly... 

Fixes https://github.com/vlakarados/gitstatus/issues/2