Switch from Cobertura to Jacoco

Cobertura doesn't support java past 8:
mojohaus/cobertura-maven-plugin#30

Setting up as per Coveralls docs:
https://github.com/trautonen/coveralls-maven-plugin#jacoco

Need to bump requireMavenVersion for Jacoco plugin per `mvn versions:display-plugin-updates`

.travis.yml

@@ -6,4 +6,4 @@ jdk:
 notifications:
   email: false
 after_success:
-  - mvn clean cobertura:cobertura coveralls:report
+  - mvn clean test jacoco:report coveralls:report

pom.xml

@@ -99,24 +99,16 @@
         </executions>
       </plugin>
       <plugin>
-        <groupId>org.codehaus.mojo</groupId>
-        <artifactId>cobertura-maven-plugin</artifactId>
-        <version>2.7</version>
-        <configuration>
-          <check>
-            <branchRate>50</branchRate>
-            <lineRate>70</lineRate>
-            <haltOnFailure>true</haltOnFailure>
-            <totalBranchRate>50</totalBranchRate>
-            <totalLineRate>70</totalLineRate>
-            <packageLineRate>70</packageLineRate>
-            <packageBranchRate>50</packageBranchRate>
-          </check>
-          <formats>
-            <format>html</format>
-            <format>xml</format>
-          </formats>
-        </configuration>
+        <groupId>org.jacoco</groupId>
+        <artifactId>jacoco-maven-plugin</artifactId>
+        <version>${jacoco.version}</version>
+        <executions>
+          <execution>
+            <goals>
+              <goal>prepare-agent</goal>
+            </goals>
+          </execution>
+        </executions>
       </plugin>
       <plugin>
         <groupId>org.eluder.coveralls</groupId>
@@ -155,7 +147,7 @@
             <configuration>
               <rules>
                 <requireMavenVersion>
-                  <version>3.1</version>
+                  <version>3.3.9</version>
                 </requireMavenVersion>
               </rules>
             </configuration>
@@ -169,6 +161,7 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <javac.version>9+181-r4173-1</javac.version>
     <auto-value.version>1.7.4</auto-value.version>
+    <jacoco.version>0.8.6</jacoco.version>
   </properties>
 
   <dependencies>