0.8.4.0

Bugfixes:

• Produce an error message when asserting directory permissions (#250)
• Read purs version from inside the set instead of its GitHub tag (#253, #225)
• Skip copy to global cache when encountering a permissions problem (#220, #260)

Other improvements:

• Errors: add many debug logs (#251)
• CI: rewrite Curator in Haskell (#239)
• CI: build only master and tags on Travis (#247)
• Dev: add Nix section to stack.yaml (#248)
• Dev: tidy up the various executables, sources and dependencies (#251)