Add Spotify block

[mattmahn]

This PR will add support for using Spotify as a music player. It works exactly the same as the Rythmbox block.

Since I am not currently using i3/lemonbar, I have not been able to test how well it actually integrates with those. However, I have tested all the other D-Bus and Spotify functionality. The only thing that I think could be a problem is the button controls; those dbus-send commands print to stdout, so I'm not sure if that will break lemonbar or i3. When I removed the --print-reply option, then Spotify would not do what it was supposed to do (i.e. play/pause, etc). I assume that if printing to stdout is a problem, &>/dev/null could simply be tacked onto the end of the command.

Tested with:

• Spotify v1.0.25.127 from the AUR
• D-Bus daemon v1.10.8
• Arch Linux (Antergos)

P.S. I added vendor/bundle to the .gitignore, so people can keep their development and system gem environments separated.

[mattmahn]

It appears that the Travis build is failing because it cannot connect to the Docker container's D-Bus. This is a known issue: moby/moby#7459, systemd/systemd#589.

To get around this, we could use an environment variable to detect whether or not it is being tested in a Docker container, then simply not execute that code that deals with the D-Bus (its only in Barr::Blocks::Spotify#initialize).

Thoughts?

[OkayDave]

Thanks for this @mattmahn. this looks great.

For the test, I'd prefer the DBus connection to be mocked/stubbed rather than it have different behaviours for different environments. It'll keep the tests fast and consistent this way.