Revert "Travis-CI: disable clang temporarily for Coverity Scan run"

This reverts commit d859ecd21d.

.travis.yml

@@ -5,8 +5,10 @@ language: c
 # Use docker for quicker builds, it now allows https://docs.travis-ci.com/user/apt/
 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