Allow "Backward compatibility check" to fail because of the repeated false positives

Signed-off-by: William Desportes <[email protected]>

Author: williamdes

.travis.yml

@@ -11,6 +11,7 @@ matrix:
   fast_finish: true
   allow_failures:
     - php: nightly
+    - name: "Backward compatibility check"
   include:
     - php: 7.1
       name: "Coding standard check"