unblock highjson-swagger, highjson-th, hledger-api

[simonmichael]

swagger2 is in stackage, so these may be ok now.

[mihaimaruseac]

There are a few failures currently:

• highjson

  [2 of 2] Compiling Data.HighJson    ( src/Data/HighJson.hs, dist/build/Data/HighJson.o )
  
  src/Data/HighJson.hs:49:10: error:
      • Could not deduce (Semigroup (a :& b))
          arising from the superclasses of an instance declaration
        from the context: (Monoid a, Monoid b)
          bound by the instance declaration at src/Data/HighJson.hs:49:10-48
      • In the instance declaration for ‘Monoid (a :& b)’
     |
  49 | instance (Monoid a, Monoid b) => Monoid (a :& b) where
     |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  

• highjson-swagger and highjson-th blocked on highjson

[DanBurton]

I've opened an issue on highjson agrafix/highjson#2

Closing this PR. Kindly open a new one once the highjson issue has been addressed.