Add noHaddock flag to modular-artihmetic

Building of haddock for packages with UTF-8 source is already
fixed in the stdenv-updates branch.  This fixes modular-arithmetic
until that branch is merged.  Will send another pull request to
add back haddock for that branch.
Merge pull request #866 from errge/haddockutf

Add noHaddock flag to modular-artihmetic