Remove guava strong dep on javers-core

[Dufgui]

Gradle optional is sadly not optional on maven.
https://blog.gradle.org/optional-dependencies

I propose to use gradle feature variant, to put gradle as maven optional

[bartoszwalacik]

makes sense, yoda should be <optional>true</optional> in maven pom

[bartoszwalacik]

fixed in 5.8.12