Use exec for startup; no need for bash.


Fixes two issues:

* 'exec' the call to php-fpm, which means supervisor restarts will in turn kill the php-fpm master process, and all is well in the world (previously, php-fpm processes would continue running in error)
* Remove the need for /bin/bash in the startup script. This prevents shellshock based issues from affecting us, though the exec fixes that more completely too.