Eliminate warning when using with Elixir v0.15.0

Currently shows:

````
~/src/ex_rated (master ✗)$ iex -S mix
Erlang/OTP 17 [erts-6.1] [source] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

==> ex2ms
warning: the dependency ex2ms requires Elixir "~> 0.13.1 or ~> 0.14.0 or ~> 0.14.0-dev" but you are running on v0.15.1
````