Disable Travis CI OS X support

Unfortunately, Travis CI's OS X machines are 10.9.5, per
https://docs.travis-ci.com/user/osx-ci-environment/#CI-environment-OS.
While NuGet supports the RID, .NET Core does not yet publish 10.9
packages, resulting in a build failure we can't do anything about.

I've asked for best-effort support by providing runtime packages for
10.9 in dotnet/corefx#7731.

Revert "Add OS X 10.9 runtime"

This reverts commit 19ab0de84cec37bb90443df14d4e69f10c41e1b1.

PowerShellGitHubDev.psm1

@@ -36,7 +36,6 @@ function Start-PSBuild {
                      "win7-x64",
                      "win81-x64",
                      "win10-x64",
-                     "osx.10.9-x64",
                      "osx.10.10-x64",
                      "osx.10.11-x64")]
         [Parameter(ParameterSetName='CoreCLR')]