upgrade all gem dependencies (2 held back)
test with 2.1.5 on travis CI
Glob for video device.

Only implemented for Linux.  mplayer defaults to `/dev/video0`, so if
multiple devices were available, but the default one was removed, the
capture would fail even though another device was available, e.g.
`/dev/video1`.
rubocop only for >= Ruby 1.9.3
adding Ruby 2.2.0 to travis config
Merge pull request #244 from mroth/upgrade-all-gem-dependencies

upgrade all gem dependencies (2 held back)
Merge pull request #246 from Ferada/auto-video-device

Glob for default video device.
updating CHANGELOG for v0.5.7 release
version set at v0.5.7
bumping rubocop version