build failure with base < 4.9

[hvr]

As can be observed at

https://matrix.hackage.haskell.org/package/shelly@1567109168

despite advertising compatibility with base-4.9 and older, shelly-1.9.0 failed to build in those build configurations. I already revised the metadata via

http://hackage.haskell.org/package/shelly-1.9.0/revisions/

to reflect the requirements more accurately so that cabal won't be running into this invalid build plans; the result of which can be observed at

https://matrix.hackage.haskell.org/package/shelly@1567491748

please take this into account for future shelly releases.

[andreasabel]

This is fixed.