Improve user defined sass spec runs for GitHub PRs
Improve user defined sass spec runs for GitHub PRs

From now on if you have a link to a sass-spec PR well run against
that PR in CI. We'll also continue to run against sass-spec master
for the time being until we have confidence in this approach.

panicinc/libsass,FishingCactus/libsass,mtwebster/mint-libsass,FishingCactus/libsass,khchanel/libsass,mikebarkas/libsass,khchanel/libsass,mikebarkas/libsass,mikebarkas/libsass,technoAri/libsass,panicinc/libsass,errorx666/libsass,panicinc/libsass,saper/libsass,mikebarkas/libsass,panicinc/libsass,am11/libsass,technoAri/libsass,errorx666/libsass,mtwebster/mint-libsass,am11/libsass,FishingCactus/libsass,saper/libsass,technoAri/libsass,saper/libsass,mtwebster/mint-libsass,technoAri/libsass,panicinc/libsass,mtwebster/mint-libsass,am11/libsass,am11/libsass,technoAri/libsass,am11/libsass,FishingCactus/libsass,khchanel/libsass,errorx666/libsass,saper/libsass,mtwebster/mint-libsass,FishingCactus/libsass,mikebarkas/libsass,errorx666/libsass,khchanel/libsass,saper/libsass,khchanel/libsass