Switch to "Current" Ruby framework symlink

Closes Homebrew/homebrew#29795.
Closes Homebrew/homebrew#29819.

Library/Contributions/cmd/brew-server

-#!/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
+#!/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby
 
 ## brew server: Run a local webserver for browsing available and installed brews.
 # Note: this external command is ruby, but set up as a shell script, so that it gets exec'd.

Library/ENV/4.3/cc

-#!/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby -W0
+#!/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby -W0
 
-$:.unshift "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8"
+$:.unshift Dir["/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/lib/ruby/{1.8,2.0.0}"].first
 require 'pathname'
 require 'set'
 require 'stringio'

Library/brew.rb

-#!/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby -W0
+#!/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby -W0
 # encoding: UTF-8
 
 std_trap = trap("INT") { exit! 130 } # no backtrace thanks