Simplify Makefile error when OTP check fails
Build on Travis CI

Install `nettle-dev` even if there is logic in the build
infrastructure for downloading it because of build error for missing
`nettle/md5.h`.  Because of this, cannot use Travis CI container
(`sudo: false`).
Merge pull request #7 from lucafavatella/checkotp-error

Simplify Makefile error when OTP check fails
maps:* bifs added
moving to Erlang 17
Merge pull request #9 from lucafavatella/ci

Build on Travis CI
all maps:* BIFs implemented
Merge branch 'master' of github.com:cloudozer/ling