Netcore build step fix

[ncave]

• Fixed codegen build step regression.
• Added netcore target as required for the build to pass (appveyor).
• Allow osx mono:latest (mono:4.4.0) target to fail (travis ci).

[dsyme]

OK great!

[7sharp9]

@dsyme / @ncave So how can I build FCS on dotnetcore, are there prereqs other than dotnetcore?

[ncave]

On Windows, nothing else needed, just run "build DotnetCliBuild" or "build DotnetCliTests".
On osx/linux - netcore versions of FAKE/FsLex/FsYacc still WIP (maybe @enricosada can help) so mono codegen still required, but builds fine after that on dotnetcore. Build binaries are already available in the appveyor build and nuget feed (please ignore v1.0.0 as it is old, there is a more recent -alpha- version).

[enricosada]

@ncave GREAT WORK!

i added a comment #465 (comment) about the dependencies and merge of binaries, but i think is ok.

Fake i dunno atm,
the FsLexYacc need some merge from open source and visualfsharp version, so i cannot do that soon, but are extenal tools, so it's ok to run these with mono i think.

[ncave]

@7sharp9 @enricosada I've added some pending issues with the dotnet core version: #581 and #582