.gitignore: add coverage/ subdirectory
tag Guard.setup_signal_traps spec as slow

This allows these examples to be excluded via

  rspec -t ~speed:slow
include guard-rspec in test group environment since it's required by guard_spec.rb

This fixes the breakage introduced by 613b11d into Travis builds.
prevent tmux output from messing up spec output
Revert silent discard of NoMethodError from plugins introduced by 95d9cd1b