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

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
```