use mono 3.2.7 on travis

reenabled build on branches. Is useful when someone fork the project and want to use personal build on feature branch ( pre pull request )

You can blacklist branches ( if needed ) with:
```
# blacklist
branches:
  except:
    - legacy
```
Merge pull request #33 from enricosada/patch-1

use mono 3.2.7 on travis