ReleaseUpdater: refactor to use Process.send_after

[jbodah]

We had a rogue test failing in our test suite after upgrading to Elixir 1.4.0-rc.1 (currently on tzdata 0.5.9 too). After a lot of digging it seems like something is failing due to how Tasks are spawned. I couldn't figure out what the exact issue was, but this refactoring seemed to fix it.

Here's the error I was seeing which was being thrown by the Task.async calls almost immediately in our test.

16:30:27.406 [error] Tzdata.ReleaseUpdater :tzdata_release_updater received unexpected message in handle_info/2: {#Reference<0.0.3.1385>,
 %Task{owner: #PID<0.560.0>, pid: #PID<0.905.0>, ref: #Reference<0.0.5.5243>}}

16:30:27.407 [error] Tzdata.ReleaseUpdater :tzdata_release_updater received unexpected message in handle_info/2: {:DOWN, #Reference<0.0.3.1385>, :process, #PID<0.560.0>, :normal}

It looks like those are being thrown because the Task is trying to send back to the Server process after running, and the server is crashing because it's trying to parse the message as a handle_info: https://github.com/elixir-lang/elixir/blob/master/lib/elixir/lib/task/supervised.ex#L36. It's a little in the weeds for me, but I think this is generally caught by Task.await, but is being bubbled up to the GenServer implementation instead

This refactor re-uses the existing Server process instead of creating a Task process

[jbodah]

Reproducible by adding a long wait (say 7000ms) in any test in this repo

[lau]

Hi Josh. Thanks! I'll probably merge this soon after doing some testing.

[venkatd]

@lau merging this should resolve #28

[lau]

❤️ 💚 💙 💛 💜

Closes #28

[jbodah]

Thanks @lau