Slf4j 1.6 is binary compatible with 1.7 - change OSGi import

The OSGi import for SLF4J is defaulted to 1.7-2.0 by Maven. Since 1.7 is
binary compatible with 1.6, users of the Metrics API should not be required
to use 1.7+. Modify the import to indicate use of 1.6+ is acceptable.