CAS-1364: The ehcache-core dependency version 2.7.2 does not exist

It could be replaced by the ```ehcache``` dependency version 2.7.2 (same groupId : ```net.sf.ehcache```).

However, in fact, the ```net.sf.ehcache.statistics.LiveCacheStatistics``` class used by the ```cas-server-integration-ehcache``` module does not exist any more in the version 2.7.2.
To avoid too much modification, I'll propose to downgrad to version 2.6.6.