Drop using `Mix.Utils` in runtime.

Right now Mix.Utils module is being used in two places:

ex_twilio/lib/ex_twilio/url_generator.ex

Line 110 in 1752a05

module |> resource_name |> Mix.Utils.underscore
ex_twilio/lib/ex_twilio/url_generator.ex

Line 161 in 1752a05

name |> to_string |> Mix.Utils.camelize

Mix is a build tool that isn't meant to be used in a runtime, but only for compilation purposes.

When building a production release of a project that uses ex_twillio developers are forced to add

defp apps do
  [
    :logger,
    :ex_twilio,
    :mix,
  ]
end

which should not happen.

Good point. I'll need to create camelize and underscore functions, or else drop in a dependency to provide those.

This has been fixed, and the fix is included in the new 0.1.3 release.