maven@3.2: make the test Java 9 compatible

JDK 9 dropped support of source and target 1.5, however they
are the default for maven. This commit changes this.

Also it removes the warning about file encoding.

Formula/maven@3.2.rb

@@ -44,6 +44,11 @@ class MavenAT32 < Formula
         <groupId>org.homebrew</groupId>
         <artifactId>maven-test</artifactId>
         <version>1.0.0-SNAPSHOT</version>
+        <properties>
+          <maven.compiler.source>1.8</maven.compiler.source>
+          <maven.compiler.target>1.8</maven.compiler.target>
+          <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        </properties>
       </project>
     EOS
     (testpath/"src/main/java/org/homebrew/MavenTest.java").write <<~EOS