Ubuntu Trusty doesn't ship with a new enough pkg-config, which causes
the PKG_INSTALLDIR macro to fail on those systems. Since Trusty is EOL
anyway, simply disable the CI build on it.
This commit adds OSX to the build matrix, as well as building on Trusty,
Xenial and Bionic, both with GCC and CLang on all distributions. For
now, the OSX build uses the default image provided by Travis CI, but we
may decide to add more images in the future.
Also move realpath installation out of addons::apt::packages, since on
Xenial and newer distributions, realpath is included in the coreutils
package and doesn't need to be explicitly installed. However, we still
need it on Trusty, so add a line to manually install it.