Revert jeweler to 1.8.4

jeweler 1.8.4 through 1.8.7 don't work with the latest version of the
git gem.  Specifically, version bumping generates a stack trace.  See:

    https://github.com/technicalpickles/jeweler/issues/245
Add Ruby 1.8 instructions to README
Version bump to 0.7.1