Proposal: Adjust ci jobs to actual pr changes

At the moment, we run builds under 14 different environments when we do CI. I have noticed that for the vast majority of commits, this is wasted effort. Usually, the important things are to (1) test every GHC version, (2) test Windows, and (3) test Mac OS X.

So, I propose to reduce the set of configurations we test during PRs to Linux (once for every GHC version), Windows, and the latest GHC with Mac OS X. We'll continue to run all tests for commits to master, and we should have some mechanism for triggering the extra tests in PR if a user wants them (e.g., a high risk commit.)