Bump SLF4J API version

The newest version has a handful of cool features, such as the ability to detect wrongly-named loggers.
Exclude SLF4J from metrics-jersey2

Allows DW to bump SLF4J dependency version independently.
Bump SLF4J again to pick up bugfix

SLF4J 1.7.9 removes usage of @Nonnull,
which was causing problems for Scala programs.
Merge pull request #814 from punya/patch-1

Upgrade to SLF4J 1.7.9