Open.popen3 does not set $?, unless Ruby 1.8

This is possibly the cause of other glitches such as random
failures on Travis.

Ruby 1.8 popen3 does set global $? for the exit status, Ruby
1.9 and above does not.  For Ruby 2.0 users (Mavericks and
above with recent Homebrew), success or failure of the current
external command was determined by the exit status of some
previous command.

Closes caskroom/homebrew-fonts#186