Use defaultConfigWithSandbox so that we pickup $HASKELL_PACKAGE_SANDBOX....

...  This fixes issue #7
defaultConfigWithSandbox returns an IO Config so I moved the call to a separate line.
Data.Default is no longer used so I removed it.
I've only tested it on my one OSX machine, if you have concerns I can do more testing.
This is the first time I've created a pull request so please excuse any rookie mistakes.