Rename leftover fields with f-prefix

[marcphilipp]

Due to serialization issues (see #976) we have decided not to rename certain fields in classes that implement Serializable. We should increment the serialVersionUID of those classes and rename the fields in 5.0.

Here's a list of classes:

• ArrayComparisonFailure.java - junit/src/main/java/org/junit/internal
• AssumptionViolatedException.java - junit/src/main/java/org/junit/internal
• ComparisonFailure.java - junit/src/main/java/org/junit
• Description.java - junit/src/main/java/org/junit/runner
• Failure.java - junit/src/main/java/org/junit/runner/notification
• InitializationError.java - junit/src/main/java/org/junit/internal/runners
• InitializationError.java - junit/src/main/java/org/junit/runners/model
• MaxHistory.java - junit/src/main/java/org/junit/experimental/max
• MultipleFailureException.java - junit/src/main/java/org/junit/runners/model

[kcooney]

This would likely break people