haskell-packages.nix: switch default versions to aeson 0.7.x and lens 4.x
haskell-Chart: jailbreak to fix build with lens 4.x

Fixes <https://github.com/timbod7/haskell-chart/issues/21>.
haskell-lens: remove obsolete version 3.10.2
haskell-aeson: remove obsolete version 0.6.2.1