Ensure that we run an JDK8

Even though producing JDK7 byte code, JDK8 is required to compile as we
also support and test JDK8-bundled classes like `Optional`.

Author: derjust

.travis.yml

@@ -1,5 +1,11 @@
+# No fancyness - we can use the new build infrastructure
+sudo: false
+
 language:
-  java
+  - java
+
+jdk:
+  - oraclejdk8
 
 before_script:
   - pip install --user codecov

pom.xml

@@ -27,7 +27,7 @@
 
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <maven.compiler.source>1.8</maven.compiler.source>
+        <maven.compiler.source>1.7</maven.compiler.source>
         <maven.compiler.target>1.7</maven.compiler.target>
     </properties>
 
@@ -138,6 +138,25 @@
     <build>
         <pluginManagement>
             <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-enforcer-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <id>enforce-java</id>
+                        <goals>
+                            <goal>enforce</goal>
+                        </goals>
+                        <configuration>
+                            <rules>
+                                <requireJavaVersion>
+                                    <version>1.8.0</version>
+                                </requireJavaVersion>
+                            </rules>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
             </plugins>
         </pluginManagement>
         <plugins>