update for Elixir 0.14.x

bump Elixir constraint in mix.exs
use on_exit/2 instead of removed teardown/1 function in tests
disable linking in meck to avoid crashes during teardown
update travis for elixir 0.14
update httpoison dependency constraint
rollback to 17.0 in travis matrix

no OTP 17.1 on Travis yet
add hackney Github dependency
Merge pull request #11 from reset/elixir-0-14

update for Elixir 0.14.x