Protect against NoClassDefFoundError on WAS

Update HibernateJpaAutoConfiguration to catch NoClassDefFoundError when
setting the JTA_PLATFORM. The exception can occur when running on WAS
since it ships with Hibernate 4.2 and SpringJtaPlatform extends from
AbstractJtaPlatform which is not present.

The exception is now ignored if a JDNI environment is available,
otherwise it is re-thrown.

Fixes gh-2218