Fix for #25

    GHC-7.6 have bug (#8072) which results in calculation of wrongs
    number of buckets in function `uniformRange'. Consequently unformR
    generates values in wrong range.
    
    Bug only affects 32-bit systems and Int/Word data types. Word32
    works just fine. So we set Word32 as unsigned counterpart for Int
    and Word on 32-bit systems. It's done only for GHC-7.6 because
    other versions are unaffected by the bug and we expect that GHC may
    optimise code which uses Word better.

@bos please merge this and release new version as soon as possible. This issue makes mwc-random broken on 32-bit linux with GHC-7.6.3. And it's unlikely that we will get 7.6.4 with bug fix.