Upgrade httpotion to v0.13 friendly version

Signed-off-by: chatgris <jboyer@af83.com>
Relax Elixir verison in mix.exs and fix incompatibilities

Signed-off-by: chatgris <jboyer@af83.com>
Update travis.yml to run test with Elixir master

Signed-off-by: chatgris <jboyer@af83.com>