Build failure with ghc 8.4

```
Building library for text-format-0.3.1.1..
[1 of 7] Compiling Data.Text.Format.Functions ( Data/Text/Format/Functions.hs, .stack-work/dist/x86_64-osx/Cabal-2.2.0.0/build/Data/Text/Format/Functions.o )

/Users/dan/scratch/text-format-0.3.1.1/Data/Text/Format/Functions.hs:16:7: error:
Ambiguous occurrence ‘<>’
It could refer to either ‘GHC.Base.<>’,
imported from ‘GHC.Base’ at Data/Text/Format/Functions.hs:22:1-15
or ‘Data.Text.Format.Functions.<>’,
defined at Data/Text/Format/Functions.hs:32:1
|
16 | (<>)
| ^^^^
```

Presumably #21 addresses this.