We require Xcode 9 for the Swift target, and the C++ and .NET tests
shouldn't care either way.
We no longer need to install cmake: this is already in the Travis
Xcode 9.0 image (and it barfs if we try to install it again).
We no longer need the rvm bug workarounds; a newer version of rvm is
in the Xcode 9.0 image so this is no longer a problem.
The openssl package is already installed and up-to-date now, so there is
no need to install that.
These changes mean that we don't need before-install-osx-cpp.sh
or before-install-os-swift.sh at all, and before-install-osx-dotnet.sh
gets a bit shorter. .travis.yml gets a small change to handle the
fact that some platform / target combos don't need before-install any more.
cfb0041328