Summary: This was previously set to disallow trailing commas on function arguments which prettier would add. Since we are dropping support for node < 8, we can now change this setting to require trailing commas everywhere and be consistent with the prettier config at Facebook.

Reviewed By: hramos

Differential Revision: D7961098

fbshipit-source-id: fbe49513183a8c8e027bb05467a767235dbfb15b