Upgrade from findbugs 3.0.4 to spotbugs 3.1.1

Also suppress a few warnings.  Release notes:

https://github.com/spotbugs/spotbugs/blob/3.1.1/CHANGELOG.md

pom.xml

@@ -216,12 +216,12 @@
         </configuration>
       </plugin>
       <plugin>
-        <groupId>org.codehaus.mojo</groupId>
-        <artifactId>findbugs-maven-plugin</artifactId>
-        <version>3.0.4</version>
+        <groupId>com.github.spotbugs</groupId>
+        <artifactId>spotbugs-maven-plugin</artifactId>
+        <version>3.1.1</version>
         <configuration>
           <effort>Max</effort>
-          <omitVisitors>FindNullDeref</omitVisitors>
+          <omitVisitors>CrossSiteScripting,DefaultEncodingDetector,FindNullDeref</omitVisitors>
         </configuration>
       </plugin>
       <plugin>
@@ -323,6 +323,12 @@
       <artifactId>jackson-dataformat-xml</artifactId>
       <version>2.9.3</version>
     </dependency>
+    <dependency>
+        <groupId>com.github.spotbugs</groupId>
+        <artifactId>spotbugs-annotations</artifactId>
+        <version>3.1.1</version>
+        <scope>provided</scope>
+    </dependency>
     <dependency>
       <groupId>com.google.code.findbugs</groupId>
       <artifactId>jsr305</artifactId>

src/main/java/org/gaul/s3proxy/ChunkedInputStream.java

@@ -31,6 +31,9 @@ final class ChunkedInputStream extends FilterInputStream {
     private byte[] chunk;
     private int currentIndex;
     private int currentLength;
+    @edu.umd.cs.findbugs.annotations.SuppressFBWarnings(
+            value = "URF_UNREAD_FIELD",
+            justification = "https://github.com/gaul/s3proxy/issues/205")
     private String currentSignature;
 
     ChunkedInputStream(InputStream is) {