Dependency version mismatch in Travis CI build

[losfair]

$ sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install llvm-6.0 llvm-6.0-dev
Reading package lists...
Building dependency tree...
Reading state information...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
 llvm-6.0 : Depends: llvm-6.0-runtime (= 1:6.0.1~svn329487-1~exp1~20180408152358.68) but it is not going to be installed
            Depends: libllvm6.0 (>= 1:6.0~svn298832-1~) but it is not going to be installed
            Depends: libstdc++6 (>= 4.9) but 4.8.4-2ubuntu1~14.04.3 is to be installed
 llvm-6.0-dev : Depends: libllvm6.0 (= 1:6.0.1~svn329487-1~exp1~20180408152358.68) but it is not going to be installed
                Depends: libstdc++6 (>= 4.9) but 4.8.4-2ubuntu1~14.04.3 is to be installed
E: Unable to correct problems, you have held broken packages.