rP3e38579feea3599043eb25897c4d0695e6411c17 bumped the PHP requirement from 5.2 to 5.5.

https://we.phorge.it/source/arcanist/browse/master/src/error/PhutilProxyException.php seems only to exist to handle PHP <= 5.3.0.

There are nearly 80 calls/instances across the arcanist and phorge codebases.