Actually, net.sf.ehcache:ehcache is the older one: http://mvnrepository.com/artifact/net.sf.ehcache/ehcache links to http://mvnrepository.com/artifact/org.ehcache/ehcache. It is true that the older version has a more recent update (April this year), but I wouldn't say that org.ehcache:ehcache is not updated; it's got an update as recent as March this year. That's only a month before the old version, and that was probably a bugfix for people still using the old version. If anything, I'd say that org.ehcache:ehcache is updated more often - the last updates were March, March and February (all this year), whereas net.sf.ehcache:ehcache's latest updates were April this year and April last year.
You should of course use version 3.5.2 instead of 3.4.0, that's true.