Rails4: add "test:rails4units", "test:rails4functionals" and "test:rails4integration" rake tasks

Rails4 "test:units" does not run "test/unit/*.rb".

--HG--
branch : rails4.20140608-0
extra : rebase_source : 993b5391f7cecb1d9ae5b1e57e52669049986f6c
.travis.yml: use rails4* tests

--HG--
branch : rails4.20140608-0
extra : rebase_source : e7733dc47e6ba22a441ff6cbecc12cb7452a86bc