Swap mpc out for ruby-mpd

[ruby-mpd](https://github.com/archSeer/ruby-mpd) has gotten a lot more mature since we looked at the state of libraries last year. Makes sense to switch, for a lot of reasons. Here's the benchmark comparison:

```
2.0.0-p195 in play/ on v3
› script/bm
                            user     system      total        real
PlayQueue.now_playing   0.290000   0.780000   3.310000 (  6.549815)
Song.find               4.350000   2.530000   9.620000 ( 18.218524)
Artist.all              1.790000   1.630000   6.050000 (  8.355747)

2.0.0-p195 in play/ on ruby-mpd 
› script/bm
                            user     system      total        real
PlayQueue.now_playing   1.330000   0.180000   1.510000 (  1.754545)
Song.find               4.050000   0.530000   4.580000 ( 11.256913)
Artist.all              4.690000   0.070000   4.760000 (  5.591033)
```

And here's an informal comparison of a page request on an album listing page (where we currently have iterate and instantiate over all of an artist's songs):

```
      v3: Completed 200 OK in 1259ms (Views: 997.5ms | ActiveRecord: 1.0ms | MPD: 1262.4ms)
ruby-mpd: Completed 200 OK in 816ms  (Views: 526.1ms | ActiveRecord: 5.7ms | MPD: 136.5ms)
```