travis-ci coverity properly run and submit scan

- the ignored warning (-Wno-missing-field-initializers) can be dropped in the future when GCC 4.8 is no longer supported

.travis.yml

@@ -7,20 +7,10 @@ git:
 matrix:
   fast_finish: true
   include:
-    - env: BUILD_TARGET=coverity_scan
+    - os: linux
       if: branch = coverity_scan
-      os: linux
-      addons:
-        apt:
-          sources:
-            - ubuntu-toolchain-r-test
-          packages:
-            - g++-7
-      env:
-        - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
 
 before_install:
-    - eval "${MATRIX_EVAL}"
     - echo -n | openssl s_client -connect scan.coverity.com:443 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' | sudo tee -a /etc/ssl/certs/ca-
 
 install: