Compatibility with mtl-2.3

[sjakobi]

Building library for jose-0.9..
[ 1 of 18] Compiling Crypto.JOSE.AESKW ( src/Crypto/JOSE/AESKW.hs, dist/build/Crypto/JOSE/AESKW.o, dist/build/Crypto/JOSE/AESKW.dyn_o )

src/Crypto/JOSE/AESKW.hs:76:29: error:
    Variable not in scope: join :: [[Int]] -> [b]
   |
76 |     let coords = zip [1..] (join (replicate 6 [1 .. n `div` 8]))
   |                             ^^^^

Note that some dependencies aren't compatible yet, e.g. haskell-tls/hs-certificate#129.

[frasertweedale]

I'll wait until next release of mtl-2.3.x with the fix for haskell/mtl#117, as well as for hs-certificate to unbreak (maybe the restoration of ExceptT export will suffice for that). Then I should be able to properly address this.