[csharp] Use a different URL for Mono < 4.4 on OSX
[csharp] Use a different URL for Mono < 4.4 on OSX

Mono 4.4 uses a universal (x86+x64) package now instead of a x86-only package, so the URL changed.

Fixes parts of https://github.com/travis-ci/travis-ci/issues/6239

JuliaCI/travis-build,kevmoo/travis-build,andyli/travis-build,JuliaCI/travis-build,kevmoo/travis-build,craigcitro/travis-build,andyli/travis-build,craigcitro/travis-build