Revert "stack.yaml: Update to lts-7.0 (ghc 8)"

Revert "stack.yaml: Update to lts-7.0 (ghc 8)"

This reverts commit e1816031037ea10c536daaf968522a1abba9082b.

This broke the i386ancient autobuilder due to its use of
--flag git-annex:XMPP --flag=git-annex:dbus

-- Failure when adding dependencies:
fdo-notify: needed ((>=0.3)), stack configuration has no specified version
(latest applicable is 0.3.1)
gnutls: needed ((>=0.1.4)), stack configuration has no specified version
(latest applicable is 0.2)
network-protocol-xmpp: needed (-any), stack configuration has no specified
version (latest applicable is 0.4.8)

OSX autobuilder also seems hosed by it, so too soon.

De-revert later..