Support hashable 1.5 in js backend

[alexfmpe]

Fixes #496.

The current vanilla ghc CI doesn't have a hashable constraint so 1.5 was already being picked up as a transitive dep even though the js conditional rules it out.

Turns out cabal outdated doesn't see through the platform conditional. Not sure what would work for detecting this - the obvious attempt fails

$ cabal outdated --with-compiler=javascript-unknown-ghcjs-ghc --with-hc-pkg=javascript-unknown-ghcjs-ghc-pkg
Error: cabal: unrecognized 'outdated' option
`--with-compiler=javascript-unknown-ghcjs-ghc'

unrecognized 'outdated' option
`--with-hc-pkg=javascript-unknown-ghcjs-ghc-pkg'