This PR makes developer tests parallel using test-queue:
https://github.com/tmm1/test-queue

## Before

Around 55 seconds:

```console
$ bundle exec rake spec
(snip)

Finished in 55.03 seconds (files took 0.77671 seconds to load)
865 examples, 0 failures

Randomized with seed 6253
```

## After

Around 11 seconds (e.g. 8 CPUs with hyper threading) :

```console
$ bundle exec rake spec
rspec-queue spec
Starting test-queue master (/tmp/test_queue_12223_3620.sock)

==> Summary (16 workers in 11.6051s)

    [ 1]                                      65 examples, 0 failures         1 suites in 7.1969s      (pid 12226 exit 0 )
    [ 2]                                      19 examples, 0 failures         1 suites in 7.3070s      (pid 12227 exit 0 )
    [ 3]                                      20 examples, 0 failures         1 suites in 11.5977s      (pid 12228 exit 0 )
    [ 4]                                      80 examples, 0 failures         3 suites in 11.5978s      (pid 12229 exit 0 )
    [ 5]                                     101 examples, 0 failures         3 suites in 11.5977s      (pid 12230 exit 0 )
    [ 6]                                      55 examples, 0 failures         5 suites in 11.5976s      (pid 12231 exit 0 )
    [ 7]                                      49 examples, 0 failures         4 suites in 11.5975s      (pid 12232 exit 0 )
    [ 8]                                      57 examples, 0 failures         6 suites in 11.5974s      (pid 12233 exit 0 )
    [ 9]                                      31 examples, 0 failures         6 suites in 11.5973s      (pid 12234 exit 0 )
    [10]                                      46 examples, 0 failures         4 suites in 11.5972s      (pid 12235 exit 0 )
    [11]                                      55 examples, 0 failures         5 suites in 11.5971s      (pid 12236 exit 0 )
    [12]                                      57 examples, 0 failures         6 suites in 11.5968s      (pid 12237 exit 0 )
    [13]                                      70 examples, 0 failures         6 suites in 11.5967s      (pid 12238 exit 0 )
    [14]                                      57 examples, 0 failures         5 suites in 11.5962s      (pid 12239 exit 0 )
    [15]                                      47 examples, 0 failures         6 suites in 11.5951s      (pid 12240 exit 0 )
    [16]                                      56 examples, 0 failures         6 suites in 11.5933s      (pid 12241 exit 0 )
```

It can be up to 5x faster depending on the environment.