Use org.eclipse.jdt.launching.JRE_CONTAINER instead of rtstubs15.jar

The test pass now.

There is something suspicious though. While debugging I notice that
both the Java 1.6 and Java 1.7 classes were in the classpath even though
the .classpath file doesn't explicitly set it. This could be trouble in
the future since Wala doesn't support the new Java 1.7 bytecode format.

See #39.