Travis-CI: disable clang for Coverity run

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.travis.yml

@@ -8,7 +8,6 @@ sudo: false
 # Test build with both GCC and Clang (LLVM)
 compiler:
   - gcc
-  - clang
 
 # The secure: declaration is the encrypted COVERITY_SCAN_TOKEN, created
 #   via the "travis encrypt" command using the project repo's public key