Fixes the "reader not in scope" error

This fixes the error:

[10 of 10] Compiling Main             ( main.hs, dist/build/yesod/yesod-tmp/Main.o )

main.hs:192:35:
    Not in scope: ‘reader’
    Perhaps you meant one of these:
      ‘header’ (imported from Options.Applicative),
      ‘readIO’ (imported from Prelude), ‘readLn’ (imported from Prelude)
cabal: Error: some packages failed to install:
yesod-bin-1.2.12.5 failed during the building phase. The exception was:
ExitFailure 1