add other specific `osx_image`
add other specific `osx_image`

The default Mac OS image is not compatible with .NET Core
see https://travis-ci.org/octokit/octokit.net/jobs/160036868
adamralph/octokit.net,dampir/octokit.net,shiftkey/octokit.net,thedillonb/octokit.net,shiftkey-tester/octokit.net,alfhenrik/octokit.net,TattsGroup/octokit.net,eriawan/octokit.net,ivandrofly/octokit.net,shiftkey-tester/octokit.net,shiftkey-tester-org-blah-blah/octokit.net,eriawan/octokit.net,M-Zuber/octokit.net,ivandrofly/octokit.net,editor-tools/octokit.net,thedillonb/octokit.net,octokit/octokit.net,TattsGroup/octokit.net,dampir/octokit.net,editor-tools/octokit.net,khellang/octokit.net,khellang/octokit.net,M-Zuber/octokit.net,rlugojr/octokit.net,alfhenrik/octokit.net,shiftkey/octokit.net,shiftkey-tester-org-blah-blah/octokit.net,gdziadkiewicz/octokit.net,octokit/octokit.net,rlugojr/octokit.net,gdziadkiewicz/octokit.net