Test against Racket HEAD, too.

Especially good idea now that a new release of Racket is approaching.

This does have the downside that Frog build might show a "fail"
overall, when only HEAD build is failing, and when that's due solely
to some temporary problem in Racket HEAD. Because of that, _might_
want to undo this after the new Racket release, especially if Racket
HEAD ever becomes "too pre-release-ish" for awhile. But for now, a
good thing to test.

Also: Use install-racket.sh via the GitHub release feature, not
raw.github.com.
Merge branch 'master' of github.com:greghendershott/frog

* 'master' of github.com:greghendershott/frog:
  Fix default in commit 0e810e7 for issue #54.
  Add posts-index-uri .frogrc var. Closes #54.