Delete JUnit4ClassRunner and its dependencies

`JUnit4ClassRunner` was deprecated in JUnit 4.3 (six years and nine
major releases ago) with a comment saying "This may disappear as soon as
1 April 2009". We started having some problems with running the tests in
JDK 7, and we decided to delete the class and its support classes.
Although we try very hard to maintain backwards compatibility,
`JUnit4ClassRunner` didn't support `Rule`s, it wasn't designed to be
extensible, and it was in an internal package. Please use
`BlockJUnit4ClassRunner` instead.