Disable TH for GHC <7.6

When `template-haskell' has been built against `containers' <0.5 (as is the case
with GHC <7.6), building Pontarius XMPP (that requires `containers' >= 0.5) will
force a rebuild of `template-haskell'. This seems to break GHC (and might likely
also break `template-haskell').