Updated cabal file for PVP of req

[matsubara0507]

Since your cabal file write req=0.2.*, stackage cannot update req-0.2 to req-0.3 . see: https://github.com/fpco/stackage/issues/2594

So, I updated your cabal file.

I checked that Discord.hs is can build using req-3.0 on Travis CI. see: https://github.com/matsubara0507/Discord.hs/pull/2