Require a slf4j package with correct obsoletes directive.

In previous version, the SLF4J package contained all the various
components of the library.  With 1.7.5, the monolithic package split
into subpackages but an obsoletes directive was not applied.  This patch
requires the most recent release which does provide the correct
obsoletes information.
Merge branch 'master' into awood/require-recent-slf4j
Merge pull request #518 from candlepin/awood/require-recent-slf4j

Require a slf4j package with correct obsoletes directive.