Upgrade to SpotBugs 3.1.8

Changelog:

https://github.com/spotbugs/spotbugs/blob/release-3.1/CHANGELOG.md#318---2018-10-16

pom.xml

@@ -245,7 +245,7 @@
       <plugin>
         <groupId>com.github.spotbugs</groupId>
         <artifactId>spotbugs-maven-plugin</artifactId>
-        <version>3.1.3</version>
+        <version>3.1.8</version>
         <configuration>
           <effort>Max</effort>
           <omitVisitors>CrossSiteScripting,DefaultEncodingDetector,FindNullDeref</omitVisitors>