fixed slf4j-simple dependency

slf4j-simple is not a provided dependency - it just looks like it when running mvn builds because a maven plugin uses jetty 6 which provides this logging stub. Jetty 9 and the other spark dependencies do not. 

When actually built and run standalone, there is no logging facility at all  - 

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

So there are no logging messages. This change fixes this.