Compile failure with pre-AMP `base` versions

[hvr]

Trying to compile webapp with GHC 7.8.4 (and older) fails with

Configuring component lib from webapp-0.4.1...
Preprocessing library webapp-0.4.1...
[1 of 8] Compiling Web.App.Parameter ( src/Web/App/Parameter.hs, /tmp/matrix-worker/1492010367/dist-newstyle/build/x86_64-linux/ghc-7.8.4/webapp-0.4.1/build/Web/App/Parameter.o )

src/Web/App/Parameter.hs:70:50: Not in scope: ‘<$>’

src/Web/App/Parameter.hs:71:48: Not in scope: ‘<$>’

src/Web/App/Parameter.hs:73:36: Not in scope: ‘<$>’

src/Web/App/Parameter.hs:75:50: Not in scope: ‘<$>’

src/Web/App/Parameter.hs:77:39: Not in scope: ‘<$>’

One simple way to fix this is (unless you want to support GHCs prior to AMP) is to tighten the lower bound on base to base >= 4.8.