Upgrade to slf4j 1.7.8

Changelog:

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

pom.xml

@@ -170,7 +170,6 @@
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <jclouds.version>1.8.1</jclouds.version>
-    <slf4j.version>1.7.7</slf4j.version>
   </properties>
 
   <prerequisites>
@@ -233,7 +232,7 @@
     <dependency>
       <groupId>org.slf4j</groupId>
       <artifactId>slf4j-api</artifactId>
-      <version>${slf4j.version}</version>
+      <version>1.7.8</version>
     </dependency>
   </dependencies>
 </project>