travis: Enable clang and use separate job for Coverity

.travis.yml

@@ -2,15 +2,16 @@ language: c
 compiler:
   - gcc
   - clang
+matrix:
+  include:
+    - compiler: gcc
+      env: COVERITY=1
 before_install:
+  - if [ "$COVERITY" = "1" -a "$TRAVIS_BRANCH" != "master" ]; then exit 0; fi
   - sudo apt-get update -qq
   - sudo apt-get install -qq -y libusb-dev
-script: if [ "${COVERITY_SCAN_BRANCH}" != 1 ]; then make ; fi
-
-matrix:
-  exclude:
-    - compiler: clang
-    - env: COVERITY_SCAN_BRANCH=1
+  - if [ "$COVERITY" != "1" ]; then make; exit $?; fi
+script: true
 
 env:
   global: