Bring automatic gem-rehash functionality to rbenv core

This bakes in the functionality of rbenv-gem-rehash plugin.

The Rubygems hook is improved:

- It will not rehash for gems installed in locations that rbenv otherwise
  doesn't search for binstubs; for instance in case of
  `bundle --path vendor/bundle`.
- It rescues exceptions and makes them non-lethal by warning on stderr.

rbenv.d/exec/gem-rehash.bash

@@ -0,0 +1 @@
+export RUBYLIB="${BASH_SOURCE%.bash}:$RUBYLIB"