Fix CI Hydra issue

[ksaric]

After building the project in CI, Hydra fails with:

unpacking sources
unpacking source archive /nix/store/adxhfmnr377x33505ilvhxbgl1fgn0iq-quickcheck-state-machine-0.6.0.tar.gz
source root is quickcheck-state-machine-0.6.0
setting SOURCE_DATE_EPOCH to timestamp 1547552013 of file quickcheck-state-machine-0.6.0/quickcheck-state-machine.cabal
patching sources
configuring
Configure flags:
--prefix=/nix/store/sklrga2cyl3m7n2k98b4pc9nwkm5x048-quickcheck-state-machine-0.6.0-lib-quickcheck-state-machine lib:quickcheck-state-machine --package-db=clear --package-db=/nix/store/vmx5s29zjvlynzr7mmrspya38ia6a5ca-quickcheck-state-machine-0.6.0-lib-quickcheck-state-machine-config/package.conf.d --with-ghc=ghc --with-ghc-pkg=ghc-pkg --with-hsc2hs=hsc2hs --with-gcc=cc --with-ld=ld --with-ar=ar --with-strip=strip --disable-executable-stripping --disable-library-stripping --docdir=/nix/store/hbmabl3hnwl48qwrl18yqsrd8g7kppxh-quickcheck-state-machine-0.6.0-lib-quickcheck-state-machine-doc/share/doc/quickcheck-state-machine --enable-split-sections 
Configuring library for quickcheck-state-machine-0.6.0..
Setup: Encountered missing dependencies:
pretty-show >=1.9.5

builder for '/nix/store/q1yah5hr9rvpm8w6hsixnhz65lhhcxnh-quickcheck-state-machine-0.6.0-lib-quickcheck-state-machine.drv' failed with exit code 1

It needs to be fixed.