Merge branch 'feature/systemd_user_units' of github:dvzrv/jack2 into feature/systemd_user_units
* 'feature/systemd_user_units' of github:dvzrv/jack2:
systemd/example.conf: Fixing typo and adding further explanation and pointers to relevant documentation.
.travis.yml: Switching last build plan (old clang) to use xenial... oversaw that one.
.travis.yml: Switching to xenial for all other Linux based builds, too.
systemd/wscript: Making sure to strip trailing new lines from systemd user unit dir.
.ci/install-deps.sh: Install legacy development packages.
.ci/install-deps.sh: Testing xenial image (trusty is mega dead) to hopefully get a working systemd development package with headers to be picked up during configure.