SLING-6144 - Update logback dependency to 1.1.7

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1764582 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/pom.xml b/pom.xml
index 94fbd53..7761de1 100644
--- a/pom.xml
+++ b/pom.xml
@@ -43,7 +43,7 @@
   </scm>
 
   <properties>
-    <slf4j.version>1.6.4</slf4j.version>
+    <slf4j.version>1.7.21</slf4j.version>
     <logback.version>1.1.2</logback.version>
     <pax-exam.version>3.5.0</pax-exam.version>