make cabal work again

... by hardcoding stackage lts-18.18 versions in tests.cabal

exercises/tests.cabal

@@ -4,5 +4,21 @@ build-type: Simple
 cabal-version: >= 1.10
 
 library
-  build-depends: QuickCheck, hspec, base, containers, array, transformers, template-haskell, generic-random, JuicyPixels, HTTP, http-types, text, bytestring, wai, warp, sqlite-simple, parallel
+  build-depends: QuickCheck==2.14.2
+               , hspec==2.7.10
+               , base==4.14.3.0
+               , containers==0.6.5.1
+               , array==0.5.4.0
+               , transformers==0.5.6.2
+               , template-haskell==2.16.0.0
+               , generic-random==1.4.0.0
+               , JuicyPixels==3.3.6
+               , HTTP==4000.3.16
+               , http-types==0.12.3
+               , text==1.2.4.1
+               , bytestring==0.10.12.0
+               , wai==3.2.3
+               , warp==3.3.18
+               , sqlite-simple==0.4.18.0
+               , parallel==3.2.2.0
   default-language: Haskell2010