Make relx be compatible with Erlang 17.0-rc1

Also, run tests on Travis CI against Erlang versions:
- 17.0-rc1
- R16B03-1
- R16B03
- R16B02
- R16B01

Tests against 17.0-rc1 will be green after merging https://github.com/erlware/erlware_commons/pull/50