udpate chunk/4 to chunk_every/4

[davemenninger]

Fixes for #13

chunk/4 is deprecated. Replace with chunk_every/4.

chunk_every/2 and chunk_every/4 were introduced in Elixir 1.5, so this PR removes older versions from the Travis CI.

I've bumped the Elixir version up in mix.exs and bumped the package minor version up to 1.1.0 too.

[coveralls]

Coverage remained the same at 100.0% when pulling 6ccda80 on davemenninger:chunk-every into 75788a6 on beatrichartz:master.

[davemenninger]

It seems like some of the tests based on timer pass/fail nondeterministically.

[davemenninger]

I checked the downstream dependents: https://hex.pm/packages?search=depends%3Ahexpm%3Aparallel_stream

and some of them still support older versions of Elixir. How is this usually handled, requiring newer versions of Elixir? It seems like anything older than 1.6 isn't getting security patches anymore?