.travis.yml: use danger 4.3.1 to fix "Cannot find a merge base between danger_base and danger_head" error.

Details: danger/danger#768

Author: php-coder

.travis.yml

@@ -18,7 +18,7 @@ before_script:
       pip install --user html5validator;
       pip install --user robotframework-lint;
       if [ "$TRAVIS_PULL_REQUEST" != 'false' ]; then
-        gem install danger nokogiri --no-ri --no-rdoc;
+        gem install danger nokogiri --no-ri --no-rdoc --version 4.3.1;
       fi;
     fi