Remove >= from cabal-version
Remove >= from cabal-version

```
Warning: shake.cabal:1:28: Packages with 'cabal-version: 1.12' or later should
specify a specific version of the Cabal spec of the form 'cabal-version: x.y'.
Use 'cabal-version: 1.18'.
```
ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake