Solution: rework the bash test to avoid build failures as intended
and make it an easier one-liner

Solution: use packages on Ubuntu and brews on OSX. The packages and
the brews are always kept up to date, so it's no use to rebuild the
libsodium stable branch manually everytime.

Solution: add make test to the cmake/ci_build.sh script

Solution: establish a matrix of CI options. On one axis we have the
build system (autotools, cmake, android) and on the other axis we
have the encryption options (tweetnacl, libsodium or none).

Solution: avoid cloning libsodium, default build is tweetnacl

Solution: add builds/cmake/ci_build.sh and call it from travis.yml