Constrain the network pkg version to <2.3.1

The latest network package (2.3.1.0) requires parsec3, whereas geordi depends on parsec<3.

Fixes the following problem:

Building network-2.3.1.0...
Preprocessing library network-2.3.1.0...
[ 1 of 10] Compiling Network.URI      ( Network/URI.hs, dist/build/Network/URI.o )

Network/URI.hs:580:25:
    Couldn't match expected type `Char' with actual type `String'
    Expected type: GenParser Char () Char
      Actual type: URIParser String
    In the first argument of `notFollowedBy', namely `regName'
    In a stmt of a 'do' block: notFollowedBy regName
cabal: Error: some packages failed to install:
geordi-0 depends on network-2.3.1.0 which failed to install.
network-2.3.1.0 failed during the building phase. The exception was:
ExitFailure 1