Use actual pup brew formula

This remote URL install was added to the README in
acad6bb01c, but the formula was mainlined later, in
687ba6f4ab

Let's just use the real formula now, so that brew
maintainers can issue build fixes independently
of this repo

README.md

@@ -17,7 +17,7 @@ If you have Go installed on your computer just run `go get`.
 
 If you're on OS X, use [Homebrew](http://brew.sh/) to install (no Go required).
 
-    brew install https://raw.githubusercontent.com/EricChiang/pup/master/pup.rb
+    brew install pup
 
 ## Quick start