atomic-primops.cabal: allow base-4.7.0.1 (ghc-7.8.3)

'<=' upper bound constraint is almost never correct
(unless one workarounds some known bug, then a link would help)

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Merge pull request #34 from trofi/master

atomic-primops.cabal: allow base-4.7.0.1 (ghc-7.8.3)