This adds VS 2017 to the build matrix, plus various other small
appveyor build changes:

- conda version bumped from 3.5 to 3.6
- build newer versions/architectures/python first (i.e. VS2017/x64/3.6
  is the first build, VS2015/x86/2.7 is the last)
- stop building after a job failure: often a build failure in one
  occurs everywhere; this just stops processing jobs (freeing them up
  for other PRs) if an error is hit.

Annoyingly, appveyor doesn't allow excluding tests: i.e. the test matrix
is always dense (appveyor issue 386), so for now we'll just run
everything.  (Once appveyor issue 386 is resolved, we can come back and
cut this down to 4-5 builds).