Revisit running dialyzer as part of CI

[vanstee]

Elixir 1.3 included some improvements both for working with types and in its own spec definitions. After running dialyxir (1.4) the warnings emitted seem to be real warnings and not just wonky Elixir issues coming up to the surface, so let's give it a shot and see if we can add it as a final step to our CI builds.

• Add dialyxir to Cog
• Fix or ignore all warnings emitted
• Run mix dialyzer --halt-exit-status as part of CI builds