Define inflections only when they're not already defined

This is so that Aviator doesn’t step on ActiveSupport’s toes in case
it’s already loaded into memory.

This is not the best solution since Aviator’s implementation may
diverge from ActiveSupport’s implementation at some point. For
now, this will have to do.

Change-Id: Iaf99d9499394c4a5ce1c0141f1a68492bb2f3191