Specify maven.compiler.release version

Use `maven.compiler.release` to avoid runtime issue (see ASSERT-KTH#25).
As `maven.compiler.release` lead to the usage of `-release` `javac` option.
Removed `maven.compiler.source` and `maven.compiler.target` that are no longer useful.
Important: project cannot be build anymore with Java 8 because `javac` version 8 does not support `--release` option added by Maven property `maven.compiler.release`.

pom.xml

@@ -16,8 +16,7 @@
     <properties>
         <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.target>1.8</maven.compiler.target>
+        <maven.compiler.release>8</maven.compiler.release>
         <java.src.version>1.8</java.src.version>
         <java.test.version>1.8</java.test.version>
         <maven.javadoc.source>1.8</maven.javadoc.source>