readme/build: fix git/svn url for gyp repo.

Cloning gyp via https://git.chromium.org/external/gyp.git
throws a connection failed error. Switching to:
   git clone https://chromium.googlesource.com/external/gyp.git
   OR
   svn co http://gyp.googlecode.com/svn/trunk build/gyp

These urls were copied from github.com/indutny/bud