You are compiling your plugin with the Java 8 compiler. You are running your server with Java 7, which is not capable of reading the Java 8-format class files in your plugin.
You can either:
- Update the server to Java 8.
- Make Eclipse use the Java 7 compiler (if it's installed on your development machine) by selecting a Java 7 JDK version, in Preferences > Java > Installed JREs:
- Make Eclipse tell the Java 8 compiler to pretend to be the Java 7 compiler, by setting your "compiler compliance level" to 1.7, in Preferences > Java > Compiler:
