DATAJPA-510 - Fix regression when sorting by property of associated object.

The main problem was a bug in querydsl 3.3.2 that has been resolved resolved in querydsl 3.3.3 to which we just updated in spring-data-build via #70.
Cleaner usage of querydsl API and less brittle casting.
Added test case for sorting by nested association property that is wrapped in a function (lower(...)).