Use public AssumptionViolatedException in Assume

JUnit 4.12 created a 'public' version of the previously 'internal'
AssumptionViolatedException.

This commit upgrades Spring's Assume class to use this new public
version.

spring-core/src/test/java/org/springframework/tests/Assume.java

@@ -21,7 +21,7 @@ import java.lang.reflect.Method;
 import java.util.Set;
 
 import org.apache.commons.logging.Log;
-import org.junit.internal.AssumptionViolatedException;
+import org.junit.AssumptionViolatedException;
 
 import org.springframework.util.ClassUtils;