Bump nixpkgs and simplify/clean up nix setup (#1529)

* Refactor tests and fix README

* fix linting hints from new hlint version

* postgrest-style with new nixpkgs-fmt

src/PostgREST/App.hs

@@ -82,7 +82,7 @@ postgrest conf refDbStructure pool getTime worker =
           -- Need to parse ?columns early because findProc needs it to solve overloaded functions.
           -- TODO: move this logic to the app function
           let apiReq = userApiRequest (configSchemas conf) (configRootSpec conf) req body
-              apiReqCols = (,) <$> apiReq <*> (pRequestColumns =<< iColumns <$> apiReq)
+              apiReqCols = (,) <$> apiReq <*> (pRequestColumns . iColumns =<< apiReq)
           case apiReqCols of
             Left err -> return . errorResponseFor $ err
             Right (apiRequest, maybeCols) -> do