stack.yaml: switch to lts-13.0 (ghc 8.6.3)
stack.yaml: switch to lts-13.0 (ghc 8.6.3)

kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev