Update project to use Java 11

[rodionmoiseev]

Currently the project is Java8 source/binary compatible, however Oracle will stop actively supporting Java8 from end of 2020.

Java11 is the next LTS version, and thus seems reasonable to be the candidate for the next official platform version for c10n.

The minimal fix would be to simply bump the source+binary compatibility requirements, make sure everything compiles and executes, and that Travis is happy.

Any source code fixes to use the latest Java11 syntax should have their own home as another issue. Cheers.

[rodionmoiseev]

Due to major dependency update, c10n major version was bumped to 2.0

Here are dependencies had to be transitively updated:

• Gradle version bumped to 6.2.2
• "idea" and "eclipse" plugins for Gradle stopped working due to updates in Gradle Tooling API. However, since latest IntelliJ provides great Gradle project support, just switched to that instead. Eclipse support was scarified, however.
• Reflections and Mockito had to be bumped to latest verions (0.9.12 and 3.3.0 respectively)