Fix warnings for Elixir 1.4.0

[xtian]

I fixed a few warnings that were being output for function/variable ambiguity and the Dict module.

[xtian]

@doomspork Any further thoughts on this? As I understand it, breakages relating to the Elixir version could be introduced in two ways:

• a new API/language feature could be used which would break for old versions which didn't support it
• some old API/language feature could be deprecated in a new version (such as the function/variable ambiguity fixed in this PR)

Both cases are covered by testing the oldest and newest supported versions. Anything else would be a semver violation by the Elixir project and should be fixed there.

[doomspork]

Thank you @xtian 👍