Travis broke, try to fix it.

```
The following packages have unmet dependencies:
 git : Depends: git-man (> 2:1.8.3.4) but 2:1.8.3.1-0avh1~precise is to be installed
E: Unable to correct problems, you have held broken packages.
```

Since travis clearly installs git already, see if we can get away with
installing git only if it is absent.
Merge pull request #18 from seomoz/phs/conditionally-install-git-in-bootstrap

Travis broke, try to fix it.