CI cleanup

[rbino]

1. Dockerfile cleanup (master, on Gitlab)

• Remove unused deploy stage, convert build stage to test
• Use branch as cache key, add _build to cache
• Add mix format --check-formatted to test-ci
• Add config/ci.exs with just use Mix.Config to be able to use ci as env.
• Add dialyzex dep and configure it to use local directory for PLTs in ci env
• Add the PLTs directory to cached paths
• Add mix:test-dialyzer job to test stage, runs mix dialyzer in ci env.

2. Travis CI (master, on Github)

• Add Travis CI