Upgrade to slf4j 1.7.28

Release notes:

https://www.slf4j.org/news.html

pom.xml

@@ -309,7 +309,7 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <java.version>1.7</java.version>
     <jclouds.version>2.1.2</jclouds.version>
-    <slf4j.version>1.7.25</slf4j.version>
+    <slf4j.version>1.7.28</slf4j.version>
     <shade.prefix>${project.groupId}.shaded</shade.prefix>
     <surefire.version>2.20</surefire.version>
   </properties>