Install rebar on Elixir 1.3.0 and up (inc. pre)
Install rebar on Elixir 1.3.0 and up (inc. pre)

Resolves https://github.com/travis-ci/travis-ci/issues/6130

jhass/travis-build,andyli/travis-build,akoeplinger/travis-build,andyli/travis-build,akoeplinger/travis-build,JuliaCI/travis-build,JuliaCI/travis-build,craigcitro/travis-build,kevmoo/travis-build,jhass/travis-build,kevmoo/travis-build,craigcitro/travis-build