Use RC2 instead, which works fine

[MarcMil]

This PR also prepares for RC4 with only one test case still failing. The remaining test case has a problem related to https://github.com/EsotericSoftware/kryo/issues/654 As soon as this problem is fixed, kryo serializers can use the new RC.

[MarcMil]

The travis-ci build has the following problem (imho unrelated to the PR):

Installing oraclejdk8
$ export JAVA_HOME=~/oraclejdk8
$ export PATH="$JAVA_HOME/bin:$PATH"
$ ~/bin/install-jdk.sh --target "/home/travis/oraclejdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "BCL"
Ignoring license option: BCL -- using GPLv2+CE by default
install-jdk.sh 2019-07-17
Expected feature release number in range of 9 to 14, but got: 8
The command "~/bin/install-jdk.sh --target "/home/travis/oraclejdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "BCL"" failed and exited with 3 during .
Your build has been stopped.

[MarcMil]

I contacted the people from travis-ci. Here's their response:

Thank you for getting in touch and our apologies for the hassle here.
As Ubuntu 14.04 reached EOL on April 30th, 2019, we've been slowly rolling out changes to make Xenial builds the default instead of Trusty. How does this impact you? If you haven't explicitly set dist: trusty in your .travis.yml file, your builds may be routed to Xenial instead of Trusty. Sorry for the inconvenience.
To get a better understanding of the reasoning behind this migration, we invite you to look at our blog post: https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment.
As previously mentioned, if your builds are suddenly failing due to this change, you can explicitly set dist: trusty to route your builds back to the Trusty distribution.
Hopefully, this helps and don't hesitate to follow up if you have any questions or concerns we can address. We'd be happy to help!