Travis CI: disable -Wno-unreachable-code warnings
Travis CI: disable -Wno-unreachable-code warnings

The -Wno-unreachable-code warnings do not fit well with the old version
of Clang that Travis CI provides. They were already disabled for CI
builds. They also break the Coverity Scan support. So, disable them also
for Coverity Scan builds.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc