Travis CI: run Coverity Scan only in one job per build
Travis CI: run Coverity Scan only in one job per build

Specifically, this is (currently) the Linux clang job. Due to issues
with Coverity's gcc build, we cannot use a gcc build for this atm.

To elaborate, Coverity's gcc apparently misreports its version, leading
to compiler errors due to an false positive in ASS_DEPRECATED_ENUM
macro's version check, as can be observed here:
https://travis-ci.org/github/libass/libass/jobs/743655402
Coverity's clang build likely does so as well, but whatever clang build
is used happens to already support everything we need.

astiob/libass,MrSmile/libass,astiob/libass,ShiftMediaProject/libass,libass/libass,xbmc/libass,Akemi/libass,libass/libass,Akemi/libass,Akemi/libass,MrSmile/libass,xbmc/libass,ShiftMediaProject/libass