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
Merge pull request #183 from codygman/master

depends on unordered-containers 0.2.3.0 for HashMap Data instance