Turn on template-haskell-atomic-primops test for GHC >7.8

This change automatically turns on the template-haskell-atomic-primops
test when GHC >7.8 is used. This is a test case for issue #8 . The test
case remains disabled for <=7.6.3 due to a bug in GHC 7.6 described
in issue #10 .