DevelopmentTools.installed?: Use locate

Use locate rather than which to search for clang or gcc.
locate searches both $HOMEBREW_PREFIX/bin and /usr/bin.

Library/Homebrew/development_tools.rb

@@ -16,7 +16,7 @@ class DevelopmentTools
     end
 
     def installed?
-      which("clang") || which("gcc")
+      locate("clang") || locate("gcc")
     end
 
     def installation_instructions