Dependency updates and associated changes

1. Update all package dependencies to latest versions consistent with building with GHC 7.6.3.  Freeze dependencies in Cabal file.

2. Remove dependency on `blaze-builder`, switching to `bytestring` builder.

3. Update cases where JSON values are generated manually for data types for which derived `ToJSON` instances are also used.  This occurs in only one place (the import client) and is required so that the manually generated JSON values match the values generated by the derived `ToJSON` instances.  (Checked by examining the TH splices from `deriveJSON` with the latest `aeson` version.)

4. Add "DragonFly" as an OS type to `Distribution.Server.Framework.Instances` to match list in Cabal's `Distribution.System`.