feat: update dependencies and fix "No type class instance was found"

error after addition of MutableBuffer class

purescript-node/purescript-node-buffer#33

```
No type class instance was found for

Node.Buffer.Class.MutableBuffer t2
                                Effect

The instance head contains unknown type variables. Consider adding a type annotation.

while applying a function fromString
of type MutableBuffer t0 t1 => String -> Encoding -> t1 t0
to argument "Not Found"
while inferring the type of fromString "Not Found"
in value declaration serveFilesAndGet
```

Author: srghma

packages.dhall

@@ -2,7 +2,7 @@ let mkPackage =
       https://raw.githubusercontent.com/purescript/package-sets/psc-0.13.0-20190626/src/mkPackage.dhall sha256:0b197efa1d397ace6eb46b243ff2d73a3da5638d8d0ac8473e8e4a8fc528cf57
 
 let upstream =
-      https://github.com/purescript/package-sets/releases/download/psc-0.13.3-20190831/packages.dhall sha256:852cd4b9e463258baf4e253e8524bcfe019124769472ca50b316fe93217c3a47
+      https://github.com/purescript/package-sets/releases/download/psc-0.13.6-20200404/packages.dhall sha256:f239f2e215d0cbd5c203307701748581938f74c4c78f4aeffa32c11c131ef7b6
 
 let overrides = {=}
 

test/Hyper/Node/FileServerSpec.purs

@@ -34,7 +34,7 @@ serveFilesAndGet path =
     app = fileServer "test/Hyper/Node/FileServerSpec" on404
 
     on404 = Ix.do
-      body <- liftEffect (Buffer.fromString "Not Found" UTF8)
+      (body :: Buffer.Buffer) <- liftEffect (Buffer.fromString "Not Found" UTF8)
       writeStatus statusNotFound
       headers []
       respond body