BUGFIX: readlink requires chdir for relative links

Fixes Homebrew#2650 (although there is more work to be done).  The issue is that
path.readlink.exist? returned false for all relative links, because readlink
reads relative to the current working directory.  This caused relative
symlinks such as /usr/bin/tar to be deleted, because package mosh identified
/usr/bin as one of its container directories.

The simplest fix is not to use readlink at all, because path.exist?
will tell us what we want to know, independent of current dir.

lib/cask/pkg.rb

@@ -88,6 +88,6 @@ def _clean_ds_store(dir)
   end
 
   def _broken_symlink?(path)
-    path.symlink? && !path.readlink.exist?
+    path.symlink? and !path.exist?
   end
 end