Travis: coverage: report check-themes with separate flag (#2570)

This makes sense in general (every check target should get its own
flag), and might help with tracking down unexpected coverage changes
(although those appear to be related to the C code only (gcov)).

.travis.yml

@@ -187,8 +187,9 @@ script:
         make check-unit || exit 1
         do_codecov unittests
         make check-integration || exit 1
-        make check-themes || exit 1
         do_codecov functionaltests
+        make check-themes || exit 1
+        do_codecov themes
         do_codecov_gcov c_code
 
         travis_fold_end