Fix serialization warnings by applying @SuppressWarnings("serial")
when appropriate.

In certain cases and for unknown reasons, a correctly-placed
@SuppressWarnings("serial") annotation will fix the warning at the
javac level (i.e. the Gradle command-line), but will produce an
"unnecessary @SuppressWarnings" warning within Eclipse. In these
cases, a private static final serialVersionUID field has been added
with the default value of 1L.