Upgrade to jclouds 1.8.0

Using a non-SNAPSHOT release will allow the first S3Proxy release.
References #7.

Release notes:

http://jclouds.apache.org/releasenotes/1.8.0/

pom.xml

@@ -143,8 +143,7 @@
 
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-    <!-- TODO: change to release version -->
-    <jclouds.version>1.8.0-SNAPSHOT</jclouds.version>
+    <jclouds.version>1.8.0</jclouds.version>
     <slf4j.version>1.7.7</slf4j.version>
   </properties>