work around bug in git 1.8.2.1 in tests

Making empty commits in tests blocked by opening the default text
editor, waiting for a message which we specified to be empty.

http://thread.gmane.org/gmane.comp.version-control.git/225529
bail out early if readlink is not available

readlink comes from GNU coreutils.  On systems without it, rbenv used to
spin out of control when it didn't have readlink or greadlink available
because it would re-exec the frontend script over and over instead of the
worker script in libexec.

Fixes #389
suggest cloning rbenv via https instead of git://