Upgrade to slf4j 1.7.23

Release notes:

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

pom.xml

@@ -262,7 +262,7 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <java.version>1.7</java.version>
     <jclouds.version>2.0.0</jclouds.version>
-    <slf4j.version>1.7.21</slf4j.version>
+    <slf4j.version>1.7.23</slf4j.version>
     <surefire.version>2.19.1</surefire.version>
   </properties>