Configure travis to test linux and osx

[f34nk]

Travis runs build on linux per default. The setting language: elixir expects erlang to be available. Adding osx to the list shows that travis is lacking support for elixir/erlang projects on osx. Travis console:

0.04s$ wget -o ${TRAVIS_HOME}/erlang.tar.bz2 ${archive_url}
The command "wget -o ${TRAVIS_HOME}/erlang.tar.bz2 ${archive_url}" failed and exited with 8 during .