Update homebrew url

https://github.com/tubbo/chruby-default-gems/archive/v1.0.1.tar.gz doesn't exist
add a version field so homebrew doesn't complain

just using a garbage number for now...
update sha1 to reflect tubbo's master
stay out of users home directories

It's impolite to screw with someone else's personal area.
(besides, .post-install.d is a terrible name)
install the command and update caveats to match
expand DEFAULT_GEMFILE when called, not when sourced