pom: update nfs4j to version 0.18.1

Motivation:
minor bugfix release to address regression introduced with 0.18.0

Changelog for nfs4j-0.18.0..nfs4j-0.18.1
* [638910f] [maven-release-plugin] prepare for next development iteration
* [3ad8698] nfs4: check for invalid attribute prior VERIFY/NFERIVY
* [b89a200] [maven-release-plugin] prepare release nfs4j-0.18.1

Acked-by: Jürgen Starek
Target: master

pom.xml

@@ -741,7 +741,7 @@
         <dependency>
             <groupId>org.dcache</groupId>
             <artifactId>nfs4j-core</artifactId>
-            <version>0.18.0</version>
+            <version>0.18.1</version>
             <exclusions>
                 <exclusion>
                     <groupId>org.dcache.common</groupId>