guard 2.0
change gemspec dependency
update travis.yml, drop 1.8.7, upgrade 1.9.2 to 1.9.3, add 2.1.0
update readme about avoid multiple autoruns
Merge branch 'killphi-upgrade_to_guard_ver_2'

* killphi-upgrade_to_guard_ver_2:
  update readme about avoid multiple autoruns
  update travis.yml, drop 1.8.7, upgrade 1.9.2 to 1.9.3, add 2.1.0
  change gemspec dependency
  guard 2.0