fix_install_names: use Mach-O methods to find dylibs

Some libraries do not have the .dylib extension (e.g. Qt framework
libs), but need to have their install names rewritten to prevent other
packages from breaking due to upgrades. Use the new Pathname#dylib?
instead.

Library/Homebrew/keg_fix_install_names.rb

@@ -50,7 +50,7 @@ class Keg
     if (lib = join 'lib').directory?
       lib.find do |pn|
         next if pn.symlink? or pn.directory?
-        dylibs << pn if pn.extname == '.dylib'
+        dylibs << pn if pn.dylib?
       end
     end
     dylibs