If a default Ruby is set, switch to it

Set PATH variables to allow gems such as Bundler to be found and for the
`ruby-info` function to properly work.

A default Ruby must be set in ~/.ruby-version; for more information, see
https://github.com/postmodern/chruby#default-ruby.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>

modules/ruby/init.zsh

@@ -28,6 +28,9 @@ elif (( $+commands[chruby-exec] )); then
   source "${commands[chruby-exec]:h:h}/share/chruby/chruby.sh"
   if zstyle -t ':prezto:module:ruby:chruby' auto-switch; then
     source "${commands[chruby-exec]:h:h}/share/chruby/auto.sh"
+
+    # If a default Ruby is set, switch to it.
+    chruby_auto
   fi
 
 # Prepend local gems bin directories to PATH.