Travis: upload the coverage data as part of the script (#6877)

According to Travis docs the exit code of after_success doesn't affect
the build result. Instead, uploading the coverage data as the last step
of the script will cause the job to fail if the command exits with
non-zero.

https://docs.travis-ci.com/user/customizing-the-build/#Breaking-the-Build

.travis.yml

@@ -149,10 +149,9 @@ before_script:
 #=============================================================================
 # Building
 #=============================================================================
-script: share/spack/qa/run-$TEST_SUITE-tests
-
-after_success:
-  - codecov --env PY_VERSION
+script:
+  - share/spack/qa/run-$TEST_SUITE-tests
+  - if [[ "$COVERAGE" == "true" ]]; then codecov --env PY_VERSION --required ; fi
 
 #=============================================================================
 # Notifications