Create output file by gulp, not `psc` command

Fixes a bug https://github.com/purescript-contrib/gulp-purescript/pull/3#issuecomment-43334351.

The `output` option given to `psc` function should be passed to gulp, not `psc` command.
If it was passed to `psc` command, the file will be created and gulp won't receive any input stream from this function.