History/background:
1. Originally, used curl with raw.github.com to get the latest version
of install-racket.sh.
2. But GitHub discourages using raw.github.com. So switched to using the
GitHub "releases" feature to make install-racket.sh available. But
that's a PITA: (a) Need to make each release, and furthermore (b)
there's no URL meaning "the latest version of the file to be
downloaded" for the .travis.yml scripts to use, so they tediously need
to be updated to fetch using the new URL for the new version.
3. OK, so now -- how about simply cloning the GitHub repo locally, and
using the travis-racket.sh from that (which of course will be the latest
version).
This is a better example for people to use, than using raw.github.com.
Had changed this temporarily while working on the
`RACKET_VERSION=HEAD` feature. Switching back now that I just rolled
it into a new v0.3.
Although that's the default for release builds, the nightly builds
default to e.g. /usr/racket-5.90.0.9. Although that default makes sense
where people are doing side-by-side installs, we're doing an install on
a throw-away Travis VM. More basically, we need the build scripts to
find stuff consistently in /usr/racket regardless of whether this is a
release or HEAD.
This uses a nightly build installer.
As the comments in install-racket.sh state, this has the weakness of
hardcoding a version like 5.90.0.9 because AFIK there is no abstract URL
meaning "whatever the latest version is".
Possibly a simpler environment. In any case, `language: racket` was a
"no-op documentation" resulting in some default, since Racket not
officially supported. I _think_ the default was Ruby, so trying C as a
maybe simpler/cleaner base.
