Build failure with ghc 8.4

[DanBurton]

Add semigroup instances.
See also: https://ghc.haskell.org/trac/ghc/wiki/Migration/8.4#SemigroupMonoidsuperclasses

[ 4 of 25] Compiling Network.PagerDuty.Internal.Types ( src/Network/PagerDuty/Internal/Types.hs, dist/build/Network/PagerDuty/Internal/Types.o )

src/Network/PagerDuty/Internal/Types.hs:56:25: error:
    • No instance for (Semigroup (CSV a))
        arising from the 'deriving' clause of a data type declaration
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    • When deriving the instance for (Monoid (CSV a))
   |
56 |     deriving (Eq, Show, Monoid)
   |                         ^^^^^^