Add support for Java 8 in our build.

We explicitly check for supported Java versions in our build.
Add Java 8 to the list of supported versions.
Work-around for partest failure due to JDK bug.

JDK7 and JDK8 have a bug that causes JVM to print some warning about
applets that is unrelated to whatever we are testing in Scala. Let's
work it around using test/files/filters mechanism that allows us to filter
out output lines based on regexp.
Make abstract-report2 test less dependent on std library.

Replace the use of `Ordering` by custom, dummy trait Xyz defined in test's
source.

By not inheriting from Ordering in abstract-report2 we make the test
less dependent on both Scala and Java library. The reason we are less
dependent on Java is because Ordering extends Java's comparator.

This change is motivated by the fact that Java 8 introduced default
method `reversed` to Comparator interface and we get a failure due to
conflicting inheritance:

-abstract-report2.scala:9: error: trait Bippy inherits conflicting members:
-  method reversed in trait TraversableOnce of type => List[(T2, String)]  and
-  method reversed in trait Comparator of type ()java.util.Comparator[T3]
-(Note: this can be resolved by declaring an override in trait Bippy.)
-trait Bippy[T1, T2, T3] extends Collection[T1] with TraversableOnce[(T2, String)] with Ordering[T3]
-      ^
Merge pull request #3404 from gkossakowski/java8-support

Preliminary support for building and testing with Java 8