Fix dependency issue for the rbenv versions symlink
Don't build with libgmp
Use RUBY_CONFIGURE_OPTS properly

CONFIGURE_OPTS applies to every package that ruby-build compiles,
including libyaml and openssl. While those packages happily ignored the
bad options, this is just a bit better.
Merge pull request #90 from boxen/minor-fixups

Minor fixups for the 8.0.0 release