Solution: run make with -j5, as now the tests support it.

Solution: don't, libzmq's CI tests are not responsible for testing
libsodium stable releases. Save some time in the CI.

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: pass built-root when calling coveralls, to help it find the
right path to the source code.

Solution: add new coverage build, and upload results to coveralls.io