Simplify Makefile error when OTP check fails
List OTP 17 as dep in INSTALL.md

With OTP R16* the `checkotp` target fails.
Replace recursive calls to `make` with `$(MAKE)`

Ref: https://www.gnu.org/software/make/manual/make.html#Recursion
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 remote-tracking branches 'remotes/origin/ci', 'remotes/origin/checkotp-error', 'remotes/origin/fix-otp-dep' and 'remotes/origin/refactor-make-recursive-calls' into HEAD