Workaround Ruby 2 GC bug

It seems that Ruby 2 doesn't like it if the watcher.to_io ever gets
GC'd. It segfaults, see e.g.
https://travis-ci.org/jonleighton/spring/jobs/5174559.

I will try to produce a proper trace and report the bug, but in the mean
time this workaround seems to do the trick.