If you're building with Java version 7 and trying to run it on Java 6 - or really anything earlier than the version of Java you compiled with - you'll get this UnsupportedClassVersionError. Compiled Java bytecode is forwards-compatible with new releases of the JVM, but not backwards compatible. Put Java 7 on your runtime environment, or compile using JDK 6.
Specifically, from Oracle's documentation of the Java 7 release:
Binary Compatibility
Java SE 7 is binary-compatible with Java SE 6 except for the incompatibilities listed below. Except for the noted incompatibilities, class files built with the Java SE 6 compiler will run correctly in Java SE 7.
The class file version for Java SE 7 is 51, as per the JVM Specification, because of the invokedynamic byte code introduced by JSR 292. Version 51 class files produced by the Java SE 7 compiler cannot be used in Java SE 6.