Only require Cabal >= 1.10

Closes #42.
allow containers >= 0.4.2 (and fall back to Data.IntMap instead of .Strict)

It's possible this may cause degraded performance when building
against containers 0.4; presumably you are using Data.IntMap.Strict
for a reason.  Nonetheless this allows JuicyPixels to build on older
platforms.
Merge pull request #43 from byorgey/master

Small tweaks allowing JuicyPixels to build in older environments