Support minitest >= 4.7

Add minitest 5 support. refs #7 

**To test**

```
$ bin/minitest-queue ./test/sample_test.rb
$ bin/minitest-queue ./test/sample5_test.rb
```

**Caveats**

With minitest 4.7, worker works mostly same numbers of tests:

```
==> Summary (8 workers in 1.0207s)

[ 8]         4 tests, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 0.7639s      (pid 11762 exit 0)
[ 2]         4 tests, 4 assertions, 1 failures, 0 errors, 0 skips         4 suites in 0.7784s      (pid 11754 exit 1)
[ 5]         4 tests, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0159s      (pid 11757 exit 0)
[ 7]         4 tests, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0157s      (pid 11761 exit 0)
[ 6]         4 tests, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0164s      (pid 11758 exit 0)
[ 4]         4 tests, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0174s      (pid 11756 exit 0)
[ 3]         4 tests, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0180s      (pid 11755 exit 0)
[ 1]         4 tests, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0196s      (pid 11753 exit 0)
```

But, with minitest >= 5.0, worker works differently:

```
==> Summary (8 workers in 1.2791s)

[ 8]          1 runs, 1 assertions, 0 failures, 0 errors, 0 skips         1 suites in 0.2660s      (pid 55248 exit 0)
[ 2]          2 runs, 2 assertions, 0 failures, 0 errors, 0 skips         3 suites in 0.5198s      (pid 55242 exit 0)
[ 7]          4 runs, 4 assertions, 1 failures, 0 errors, 0 skips         4 suites in 0.7816s      (pid 55247 exit 1)
[ 6]          4 runs, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0220s      (pid 55246 exit 0)
[ 1]          5 runs, 5 assertions, 0 failures, 0 errors, 0 skips         5 suites in 1.2749s      (pid 55241 exit 0)
[ 5]          5 runs, 5 assertions, 0 failures, 0 errors, 0 skips         6 suites in 1.2756s      (pid 55245 exit 0)
[ 4]          6 runs, 6 assertions, 0 failures, 0 errors, 0 skips         6 suites in 1.2764s      (pid 55244 exit 0)
[ 3]          5 runs, 5 assertions, 0 failures, 0 errors, 0 skips         6 suites in 1.2771s      (pid 55243 exit 0)
```

<del>I'm working to improve minitest 5 concurrency. (It might be related to minitest's implementation. I don't know well)</del>

ada0ca8 fixed above issue. Now it works as expected.

```
$ bundle exec minitest-queue ./test/sample5_test.rb 
Starting test-queue master (/tmp/test_queue_49467_70324641115340.sock)

==> Starting test-queue worker [6] (49520 on macbook.local) - iterating over /tmp/test_queue_49467_70324641115340.sock

Run options: --seed 30445

# Running:

    MiniTestSleep14: .  <0.251>
    MiniTestSleep12: .  <0.251>
    MiniTestSleep15: .  <0.251>
    MiniTestFailure: F  <0.010>


Finished in 0.768493s, 5.2050 runs/s, 5.2050 assertions/s.

  1) Failure:
MiniTestFailure#test_fail [/Users/ozaki/Projects/Ruby/gems/test-queue/test/sample5_test.rb:21]:
Expected: 0
  Actual: 1

4 runs, 4 assertions, 1 failures, 0 errors, 0 skips

==> Summary (8 workers in 1.0265s)

    [ 8]          4 runs, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 0.7697s      (pid 49522 exit 0)
    [ 6]          4 runs, 4 assertions, 1 failures, 0 errors, 0 skips         4 suites in 0.7807s      (pid 49520 exit 1)
    [ 1]          4 runs, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0230s      (pid 49515 exit 0)
    [ 7]          4 runs, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0215s      (pid 49521 exit 0)
    [ 5]          4 runs, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0233s      (pid 49519 exit 0)
    [ 4]          4 runs, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0238s      (pid 49518 exit 0)
    [ 3]          4 runs, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0244s      (pid 49517 exit 0)
    [ 2]          4 runs, 4 assertions, 0 failures, 0 errors, 0 skips         4 suites in 1.0249s      (pid 49516 exit 0)

==> Failures


  1) Failure:
MiniTestFailure#test_fail [/Users/ozaki/Projects/Ruby/gems/test-queue/test/sample5_test.rb:21]:
Expected: 0
  Actual: 1
```

@tmm1 Even though this PR is <del>still WIP</del> **DONE**! Could you review this?

Thank you.