Add child_spec/1

[odarriba]

Adding this version of child_spec/0 allow to use this Elixir 1.5 feature:
https://github.com/elixir-lang/elixir/blob/v1.5/CHANGELOG.md#streamlined-child-specs

So the connection can be included in the app's supervision tree by doing just:

children = [
  supervisor(MyApp.Endpoint, []),
  MyApp.InfluxConnection
]

[coveralls]

Coverage remained the same at 98.516% when pulling 2f9642f on odarriba:patch-1 into 6ee0527 on mneudert:v0.17.

[mneudert]

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