No longer necessary to use a list when only specifying one 'ADDITIONAL_PREPROCESS_CMDS'.  Instead you can just use a string.
Modified hhvm extension to add hhvm to the path.  Also uses an alias to setup hhvm so it's runnable without setting the '-c' argument each time.
Fixed bug in hhvm extension where alias command was specified wrong.
Changed from using an alias to a function to override hhvm and specify the command.