double-conversion-2.0.4.2 fails to build with GHC 7.0 - 7.6

[andreasabel]

Details below.

It seems that this is caused by lacking bounds on base and bytestring.

GHC 7.6

https://github.com/hackage-trustees/text-format/actions/runs/3703919604/jobs/6275933011#step:17:110

Data/Double/Conversion/Convertable.hs:22:18:
    Could not find module `Data.ByteString.Builder'
    It is a member of the hidden package `bytestring-builder-0.10.8.2.0'.
    Perhaps you need to add `bytestring-builder' to the build-depends in your .cabal file.

GHC 7.4 and 7.2

https://github.com/hackage-trustees/text-format/actions/runs/3703919604/jobs/6275933140#step:17:110

Data/Double/Conversion/Convertable.hs:1:33:
    Unsupported extension: InstanceSigs

GHC 7.0

https://github.com/hackage-trustees/text-format/actions/runs/3703919604/jobs/6275933389#step:17:122

Data/Double/Conversion/Convertable.hs:1:14:
    Unsupported extension: DefaultSignatures

[andreasabel]

For now, I made the revision base >= 4.7 on hackage: https://hackage.haskell.org/package/double-conversion-2.0.4.2/revisions/
I didn't investigate the root causes, there might be better ways to address this, which can always be done in a subsequent revision.

In any case, dependencies should come with lower and upper bounds.