.gemrc: add --env-shebang to install options     *

This became useful after the switch-over to chruby/ruby-install.
Bundler gets installed into ~/.gem/${ruby}/X.Y.Z, but its binstub (at
least under ruby 2.0/rubygems 2.0) is generated with the full path to
the ruby executable in its shebang.

This option is supported from at least rubygems 1.8, and bundler will
likely be the only gem installed under ~/.gem.