move Ubuntu 16.04 testing to 18.04

As preparation for a migration to Python 3, this gives us an environment where
we have Python 3.6 by default. It is also the latest image currently available
Travis. On Ubuntu 18.04, all versions of GCC < 9 are natively packaged [0-3], so
we can drop the ubuntu-toolchain-test-r PPA for these builds. We can also drop
the LLVM PPAs for Clang 4, 7, and 8 that are natively packaged [4-6]. The GCC 9
build which still needs the ubuntu-toolchain-test-r PPA needs to be specified
slightly differently because this PPA is not in Travis' safe sources list for
Bionic [7].

Note that we have left the fuzzing build on Ubuntu 16.04. This is because the
latest version of AFL (2.52b) seems somehow busted on Ubuntu 18.04. It produces
a binary that consistently crashes, complaining about a bad address to
posix_spawnp. After some extensive searching, I found a clue [8] that suggests
it is a bad interaction between AFL and GCC, but the suggestion to use afl-clang
instead does not alleviate my problem.

  [0]: https://packages.ubuntu.com/bionic/g++-5
  [1]: https://packages.ubuntu.com/bionic/g++-6
  [2]: https://packages.ubuntu.com/bionic/g++-7
  [3]: https://packages.ubuntu.com/bionic/g++-8
  [4]: https://packages.ubuntu.com/bionic/clang-4.0
  [5]: https://packages.ubuntu.com/bionic-updates/clang-7
  [6]: https://packages.ubuntu.com/bionic-updates/clang-8
  [7]: ttps://github.com/travis-ci/apt-source-safelist/issues/410
  [8]: https://www.danielstender.com/blog/afl-ubuntu1804/

Github: related to #167 "switch all Python dependencies to Python 3 in
  packaging/debian"
Github: related to #170 "optimise testing"