Updated PHP from 5.5.0 to 5.5.1, SHA256 checksum

Nice tap @tzvetkoff !

I updated php to 5.5.1 (current stable) and:
* assigned a static URL (removing the /from/this/mirror);
* updated the sha256 checksum (calculated locally with `shasum -a 256`);
* added a line reflecting the update to the readme.

Tested, built and installed successfully on OS X 10.8.4.