Upgrade Eclipse project settings for Java 8

 - Set the eclipse.jdt source and target compatibility to 1.8 in
   ide.gradle.

 - Updated the instructions in import-into-eclipse.sh to reflect minimum
   requirements for building Spring Framework 4.1 against Java 8 with
   Eclipse and STS

Issue: SPR-11831
Upgrade spring-test tests to Hibernate 4

This commit upgrades Hibernate-based integration tests in the
spring-test module to use Hibernate 4 instead of 3 and Hibernate
Validator 5 instead of 4. This streamlines and simplifies our
dependency management at the same time.

Issue: SPR-11834