update nexus plugin in an attempt to solve key not found on public nexus

Author: ericchung

pom.xml

@@ -89,7 +89,7 @@
         <maven-project-info-reports-plugin.version>2.9</maven-project-info-reports-plugin.version>
         <maven-site-plugin.version>3.6</maven-site-plugin.version>
         <maven-jxr-plugin.version>2.5</maven-jxr-plugin.version>
-        <nexus-staging-maven-plugin.version>1.6.7</nexus-staging-maven-plugin.version>
+        <nexus-staging-maven-plugin.version>1.6.8</nexus-staging-maven-plugin.version>
         <maven-gpg-plugin.version>1.6</maven-gpg-plugin.version>
     </properties>