depends on unordered-containers 0.2.3.0 for HashMap Data instance

Data instance for HashMap, which only seems to appear in 0.2.3.0, but
aeson's current dependency is only on 0.1.3.0.

Data instance for HashMap

I could only reproduce this error when trying to install lens and aeson as I described in #182

Installing version 4.0 of lens after fixing this bug uncovers a lens bug. That bug is caused because cpphs is a dependency, but not listed in the cabal file. @edwardk told me he was pushing an update however so that lens no longer depends on cpphs.

After merging this change in, lens and aeson should be able to coexist a little better ;)