Upgrade to Phalcon 1.3.1 stable, SHA1 fix

Initially trying to install 1.3.0 gave me an SHA1 mismatch:
```

Error: SHA1 mismatch
Expected: 122517118769a6d41bf5f467477c25c495eda31f
Actual: d32571364510b61eb0e7d4e6049c02ebd6425780
Archive: /Library/Caches/Homebrew/php55-phalcon-1.3.0.tar.gz
```
...and since 1.3.1 stable was released in the meantime (https://github.com/phalcon/cphalcon/releases/tag/phalcon-v1.3.1), I switched the sources to the new phalcon version.

```
> shasum cphalcon-1.3.1.tar.gz
af9f7888006432effc0f48b766dd6e8e1dde1cd5  cphalcon-1.3.1.tar.gz
```