Add Hashable instance for SizeSpec2D

This pull request:

* Adds a dependency on `hashable > 1.1` (part of the HP)
* Adds a dependency on the `DeriveGeneric` extension (since GHC 7.2)
* Adds a `Hashable` instance for `SizeSpec2D`; needed for some pending changes/reorganization to `diagrams-builder`.