Update .travis.yml to get Dialyzer working
Update .travis.yml to get Dialyzer working

  - use the correct URL for the static plt
  - example.exs files must be manually compiled to ./tmp
    to create .beam files.
  - Dialyzer runs only on ./tmp

exercism/xelixir,parkerl/xelixir,cezarykosko/xelixir