Mono build on Travis CI

closes https://github.com/scriptcs/scriptcs/issues/593
partially addresses https://github.com/scriptcs/scriptcs/issues/706

* build.sh and .travis.yml added
* the old legacy MSBuild-based package restore artifacts were still present and these were breaking the build on Travis so I removed them all
* I updated the copy of NuGet.exe in ./.nuget to the latest, 2.8.2
* NuGet.Core actually has the ID **Nuget.core** in the gallery so it gets installed to a folder with that case, but the reference in csproj file gets added with path as NuGet.Core. This breaks on *nix with its case sensitive file system. To make things work, I hacked the entry in the relevant csproj files to Nuget.Core. This is something we'll have to watch out for when upgrading to new versions of NuGet.Core/Nuget.Core but at least we'll hopefully now have a Travis CI build telling us when things aren't right :bowtie:
* references, package installations and binding redirects for the test projects were all messed up so I cleared them out and rebuilt them from scratch
* xbuild verbosity is set to normal rather than minimal because the minimal verbosity is useless when there are problems, e.g. you get error messages but it doesn't even tell you in which project the errors occur
* it's set to build on Linux right now. On Travis CI it has to be either Mac or Linux, we can't have both. Given that Mac VMs on Travis CI are scarce and builds take quite a while to get provisioned on them, I suggest we go with Linux for now and reconsider if/when we find another *nix build service to take on building on a second *nix platform

It doesn't run tests yet because there are issues with the tests currently in Mono, e.g. https://github.com/scriptcs/scriptcs/pull/734 addresses some of them. The rest can come later as part of closing out https://github.com/scriptcs/scriptcs/issues/706

So, it's rather basic at the moment. All it does is restore packages and build the solution, but... it works!!!

[![ItWorks](http://media.giphy.com/media/Va7vvCMQjFrxK/giphy.gif)](http://media.giphy.com/media/Va7vvCMQjFrxK/giphy.gif)