install: update 10.11 /usr/local instructions.

These seem to not apply for everyone on 10.11.0 any more (as explained
in Homebrew/legacy-homebrew#45387).

install

@@ -145,8 +145,10 @@ Before running the installer again please agree to the license by opening
 Xcode.app or running:
     sudo xcodebuild -license
 EOABORT
-abort <<-EOABORT if !File.directory?(HOMEBREW_PREFIX) && macos_version >= "10.11"
-OS X 10.11 & above require an additional step to write into #{HOMEBREW_PREFIX}.
+sandbox_paths_file = "/System/Library/Sandbox/Compatibility.bundle/Contents/Resources/paths"
+abort <<-EOABORT if !File.directory?(HOMEBREW_PREFIX) && macos_version >= "10.11" \
+  && !(File.exist?(sandbox_paths_file) && IO.read(sandbox_paths_file).include?("/usr/local\n"))
+Your OS X 10.11 install requires an additional step to write into #{HOMEBREW_PREFIX}.
 Please follow the steps described here and then run the install script again:
   https://git.io/vnC7w
 EOABORT