Use \\ for default args instead of //

The use of // for default args was soft-deprecated in the version 0.12.3
of Elixir and will be deprecated with warning in version 0.12.4.
In new versions \\ should be used instead.
Bump Elixir version to 0.12.3
Update .travis.yml to pull Elixir 0.12.3
Merge pull request #23 from guilleiguaran/deprecations-0-12-4

Use \\ for default args instead of //