Build failing for ghc-8.4.1

[harendra-kumar]

See this travis job:

src/WithCli/Flag.hs:19:10: error:
    • Could not deduce (Semigroup (Flag a))
        arising from the superclasses of an instance declaration
      from the context: Monoid a
        bound by the instance declaration at src/WithCli/Flag.hs:19:10-36
    • In the instance declaration for ‘Monoid (Flag a)’
   |
19 | instance Monoid a => Monoid (Flag a) where
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
cabal: Leaving directory '/tmp/cabal-tmp-4524/getopt-generics-0.13.0.1'

[soenkehahn]

@harendra-kumar: I've created #57, it'd be great if you could take a look.

[soenkehahn]

Closed via #57.