Add animal sniffer plugin to check API usages.

Wouldn't want @yawkat submitting PRs that only work on Java 8.

pom.xml

@@ -165,6 +165,26 @@
           </archive>
         </configuration>
       </plugin>
+      <plugin>
+          <groupId>org.codehaus.mojo</groupId>
+          <artifactId>animal-sniffer-maven-plugin</artifactId>
+          <version>1.13</version>
+          <executions>
+              <execution>
+                  <phase>process-classes</phase>
+                  <goals>
+                      <goal>check</goal>
+                  </goals>
+              </execution>
+          </executions>
+          <configuration>
+              <signature>
+                  <groupId>org.codehaus.mojo.signature</groupId>
+                  <artifactId>java16</artifactId>
+                  <version>1.1</version>
+              </signature>
+          </configuration>
+      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-shade-plugin</artifactId>