Allow network 2.3

This makes it possible for io-streams to work with haskell-platform
2012.4.0.0.
Get test suite running with network 2.3.

I pushed this as a separate commit in case you wanted to leave this part
of the changeset out. I've tested with GHC 7.4, and this compiles and
all tests pass.
Merge pull request #20 from snoyberg/master

Compatibility with haskell-platform 2014.2