Scope slf4j-log4j12 to provided

This lets consumers of this library choose whatever slf4j binding they wish.
As it is right now, this example project:
https://github.com/scalatra/scalatra-website-examples/tree/master/2.2/swagger-example
on startup gives an slf4j warning about multiple bindings on the classpath (log4j and logback), then log4j complains about the lack of a config file, and nothing gets logged.