Support containers 0.7 in network-transport-tests

[davidsd]

See commercialhaskell/stackage#7549

[LaurentRDC]

Tested locally using cabal test network-transport-inmemory --constraint="containers^>=0.7" (network-transport-inmemory tests use network-transport-tests) with success

[LaurentRDC]

New revision published on Hackage

[davidsd]

Awesome -- thanks for the quick fix!