Add after suite stop hook for event machine

[AlanFoster]

Let's explicitly shutdown event machine after our test suite has run.

Closes #237

[ronwsmith]

How much time does this add to the test run?

[AlanFoster]

@ronwsmith As far as I can tell there's no noticeable impact:

    user     system      total         real
0.000000   0.000000   0.000000 (  0.000951)

Measured via Benchmark.measure around the added stop method.

I don't think it's noticable if we consider how long it takes for a single test to run, i.e. running this branch locally:

$ rspec ./spec/features/examples/preflight_request_spec.rb

I, [2018-04-20T14:46:26.045360 #38771]  INFO -- : puffing-billy: Proxy listening on http://localhost:54035
2018-04-20 14:46:26 WARN Selenium [DEPRECATION] Selenium support for PhantomJS is deprecated. Use headless Chrome/Firefox or HTMLUnit instead.
2018-04-20 14:46:26 WARN Selenium [DEPRECATION] Selenium support for PhantomJS is deprecated. Use headless Chrome/Firefox or HTMLUnit instead.
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Randomized with seed 61801
.  

    user     system      total         real
0.000000   0.000000   0.000000 (  0.000951)

Finished in 6.87 seconds (files took 1.43 seconds to load)
1 example, 0 failures

Randomized with seed 61801

Shows that the shutdown time is negligible

[AlanFoster]

@ronwsmith Let me know if there's anything I can do to get my inflight PRs merged and released 🎉

[ronwsmith]

Released in 1.1.0