Add posts-index-uri .frogrc var. Closes #54.
Fix default in commit 0e810e7 for issue #54.

The default should be /index.html.

While I'm at it, improve the comments.
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.
Remove test that's N/A for Racket 5.90+

(The test relied on a particular answer that 5.3.x happens to give,
but other answers could be correct. As the test message already noted,
the test isn't really portable. Although simply deleting the test may
be lazy, I don't have a great alternative to replace it with.)
Recommend using latest Pygments.

- Recommend using `--upgrade` flag with `easy_install`.
- Mention Racket supported starting with Pygments 1.6.