Add Travis for Elixir 1.5, 1.6 and OTP 21

[samhamilton]

The compatibility matrix can be found here:

https://github.com/elixir-lang/elixir/blob/master/lib/elixir/pages/Compatibility%20and%20Deprecations.md#compatibility-between-elixir-and-erlangotp

[danhper]

Sorry for the delay and thank you.