Upgrade to JDK 14

[Toparvion]

The upgrading to JDK 14 is extracted into separate PR because of compatibility issue in SonarCloud. I hope it will be resolved soon.

To merge this PR we should:

• ensure that SonarCloud works with builds on JDK14
• revert the change in Travis badge (in README.md) to make it point to master branch

[Toparvion]

Now SonarCloud does not break the build though it still doesn't recognize JDK 14 bytecode, see this build log:

Caused by: java.lang.IllegalArgumentException: Unsupported class file major version 58
	at org.objectweb.asm.ClassReader.<init>(ClassReader.java:195)
	at org.objectweb.asm.ClassReader.<init>(ClassReader.java:176)
	at org.objectweb.asm.ClassReader.<init>(ClassReader.java:162)
	at org.sonar.java.bytecode.se.MethodLookup.lookup(MethodLookup.java:68)
	at org.sonar.java.bytecode.se.MethodLookup.lookup(MethodLookup.java:60)
	at org.sonar.java.bytecode.se.BytecodeEGWalker.execute(BytecodeEGWalker.java:316)
	at org.sonar.java.bytecode.se.BytecodeEGWalker.getMethodBehavior(BytecodeEGWalker.java:293)
	... 254 common frames omitted

This should be fixed with Sonar updating to ASM 8.0.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

80.0% Coverage
0.0% Duplication