Fix some java build warnings

src/ImageJ/plugin-ij/pom.xml

@@ -15,6 +15,7 @@
   <properties>
     <name>${project.name}</name>
     <version>${project.version}</version>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
   </properties>
   <dependencies>
     <dependency>
@@ -33,6 +34,7 @@
     <plugins>
       <plugin>
         <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.8.1</version>
         <configuration>
           <source>1.8</source>
           <target>1.8</target>