shadowjar should not be an implicit dependency

[rtyler]

When I remove the line pulling in shadowjar from the plugins repository, my gradlefile fails now.

FAILURE: Build failed with an exception.

* Where:
Build file '/usr/home/tyler/source/github/blick/build.gradle' line: 36

* What went wrong:
A problem occurred evaluating root project 'blick'.
> Could not find method shadowJar() for arguments [build_53hpiloh3ahr8sdm93qjclshj9$_run_closure5@4d3c6593] on root project 'blick'.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 17.618 secs

IMHO, since we need shadowjar to build jars properly, it should be an explicit dependency, what do you think @ysb33r ?