Move ASL to GPL

[nus]

This project uses the PlantUML library of Apache Software License and the library is added in repository.
So, the library is updated, this project is more repository size.

By moving to GPL version, the problem of increasing repository size will be solved.
GPL version library will be obtained from maven repository(https://mvnrepository.com/artifact/net.sourceforge.plantuml/plantuml/6703).

[aadrian]

@nus IMHO not need for this, since PlantUML plans to offer dependencies in ASL form too, so than we can simply declare those as dependencies.

https://github.com/plantuml/plantuml/wiki/Module-Proposal

https://mvnrepository.com/artifact/net.sourceforge.plantuml/plantuml/6703

for now just merge #9

[nus]

@aadrian
Good for your proposal and work plantuml/plantuml#47 !