Travis CI build failing

[John Sichi]

Maybe JDK 11 started enforcing errors during javadoc runs:

https://travis-ci.org/jgrapht/jgrapht/builds/658019841?utm_medium=notification&utm_source=github_status

[ERROR] src/main/java/org/jgrapht/traverse/BreadthFirstIterator.java:[176] (javadoc) JavadocMethod: Expected @return tag.
5254[ERROR] src/main/java/org/jgrapht/traverse/BreadthFirstIterator.java:[184] (javadoc) JavadocMethod: Expected @return tag.

Anyway, Dimitrios' #882 should resolve this, so I'll merge that and see if we go back to green.

[John Sichi]

And apparently we were behind the times with travis-ci.org and webhooks integration, so I've migrated that to travis-ci.com with Github Apps integration, and I think all is well now.