Updated slf4j-api & slf4j-log4j12 to 1.6.6

Tomcat7 is logging to catalina.out messages like the following:
SEVERE: A web application created a ThreadLocal with key of type
[org.apache.log4j.helpers.ThreadLocalMap] (value
[org.apache.log4j.helpers.ThreadLocalMap@3ac5b23e]) and a value of type
[java.util.Hashtable] (value [{}]) but failed to remove it when the web
application was stopped. To prevent a memory leak, the ThreadLocal has
been forcibly removed.

The proposed fix is to utilize log4j 1.2.17, which fixes this particular
Memoryleak (described in
http://logging.apache.org/log4j/1.2/changes-report.html#a1.2.17 and
https://issues.apache.org/bugzilla/show_bug.cgi?id=50486)
Merge pull request #611 from tiagojco/slf4jVersionUpgrade

Updated slf4j-api & slf4j-log4j12 to 1.6.6