ENH: Simplify the way knex is added to the path

[mmornati]

Instead of linking the knex migrator file into the bin folder

        for f in "$GHOST_INSTALL/current/node_modules/.bin/knex-migrator"*; do \
		[ -x "$f" ]; \
		ln -svf "$f" /usr/local/bin/; \
	done

I think it is easier to add the knex bin folder to the user path:
ENV PATH="${GHOST_INSTALL}/current/node_modules/knex-migrator/bin:${PATH}"

It is working and tested here