Replaced existing autoloader with one generated by composer

src/php/lib/Grpc/AbstractCall.php

src/php/lib/Grpc/BaseStub.php

src/php/lib/Grpc/BidiStreamingCall.php

src/php/lib/Grpc/ClientStreamingCall.php

src/php/lib/Grpc/ServerStreamingCall.php

src/php/lib/Grpc/UnaryCall.php

src/php/lib/autoload.php

src/php/tests/generated_code/GeneratedCodeTest.php

src/php/tests/interop/interop_client.php