Add error-prone compiler. (#259)

* Fix error-prone warning

* Add error-prone compiler.

* Suppress warning about calling getClass() on annotation.

.travis.yml

@@ -1,9 +1,13 @@
 language: java
 
 jdk:
-  - oraclejdk7
   - oraclejdk8
 
+addons:
+  apt:
+    packages:
+      - oracle-java8-installer # Updates JDK 8 to the latest available.
+
 after_success:
   - .buildscript/deploy_snapshot.sh
 

moshi/src/main/java/com/squareup/moshi/Types.java

@@ -57,7 +57,7 @@ public final class Types {
     }
     for (Annotation annotation : annotations) {
       if (jsonQualifier.equals(annotation.annotationType())) {
-        Set<Annotation> delegateAnnotations = new LinkedHashSet<>(annotations);
+        Set<? extends Annotation> delegateAnnotations = new LinkedHashSet<>(annotations);
         delegateAnnotations.remove(annotation);
         return Collections.unmodifiableSet(delegateAnnotations);
       }

moshi/src/test/java/com/squareup/moshi/TypesTest.java

@@ -209,6 +209,7 @@ public final class TypesTest {
         .containsExactly(String.class, Integer.class);
   }
 
+  @SuppressWarnings("GetClassOnAnnotation") // Explicitly checking for proxy implementation.
   @Test public void createJsonQualifierImplementation() throws Exception {
     TestQualifier actual = Types.createJsonQualifierImplementation(TestQualifier.class);
     TestQualifier expected =

pom.xml

@@ -80,11 +80,25 @@
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-compiler-plugin</artifactId>
-          <version>3.0</version>
+          <version>3.3</version>
           <configuration>
+            <compilerId>javac-with-errorprone</compilerId>
+            <forceJavacCompilerUse>true</forceJavacCompilerUse>
             <source>${java.version}</source>
             <target>${java.version}</target>
           </configuration>
+          <dependencies>
+            <dependency>
+              <groupId>org.codehaus.plexus</groupId>
+              <artifactId>plexus-compiler-javac-errorprone</artifactId>
+              <version>2.8</version>
+            </dependency>
+            <dependency>
+              <groupId>com.google.errorprone</groupId>
+              <artifactId>error_prone_core</artifactId>
+              <version>2.0.15</version>
+            </dependency>
+          </dependencies>
         </plugin>
       </plugins>
     </pluginManagement>