Execute `dnu restore` with .NET Core and use Mono 4.0.5 - upgrade Linux to Trusty (Ubuntu 14.04) distribution, where .NET Core is supported - Mono beta is now 4.2.1 which doesn't work with `nuget.exe`   - see also aspnet/External#48
Execute `dnu restore` with .NET Core and use Mono 4.0.5
- upgrade Linux to Trusty (Ubuntu 14.04) distribution, where .NET Core is supported
- Mono beta is now 4.2.1 which doesn't work with `nuget.exe`
  - see also aspnet/External#48

Note: cannot use `KOREBUILD_TEST_DNXCORE=true` due to #577

aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore