Update README.md

The previous link (http://closure-compiler.googlecode.com/files/compiler-latest.zip) only contains a README file which indicates that the link is no longer valid, and gives another url to get the actual compiler-latest.zip file (which is http://dl.google.com/closure-compiler/compiler-latest.zip).
Merge pull request #317 from pmauduit/patch-1

Update README.md