use mono 3.2.7 and fsharp 3.1.1.1 on travis

build and install fsharp 3.1
this add +10, +15 min to build, but is ok

fix #447

Build fails, i think error was introduced with #435 ( 5d9215f067ac7f17679836886569fc31f0080b43 ), the first failing build, not because of user quota.

To test my conjecture:

I created a new branch [fsharp31_on_last_successful_build](https://github.com/enricosada/FSharp.Data/tree/fsharp31_on_last_successful_build) based on  [last travis successful build](https://travis-ci.org/fsharp/FSharp.Data/builds/19287979) ( 2efe9cbc4cd00def44cdfe8d42f3ab258480a43d ) and cherry-picked my changes into it.
The [build](https://travis-ci.org/enricosada/FSharp.Data/builds/19881494) fail on signature test, *after* compile error.

I can repro locally, i'll try to fix it, but maybe a revert will be easier