add support PostCSS 5.x

[DenisIzmaylov]

Hi! npm could not install this package to my project with postcss 5.0.3:

npm ERR! peerinvalid The package postcss@5.0.3 does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer css-modules-require-hook@1.0.4 wants postcss@4.x

[mightyaleksey]

Hi, I tried to update it to the 5 version yesterday, but looks like plugins should be updated first :)
So I downgraded it :)

Anyways, all tests pass with the version 5 and if you don't mind about warnings - you can use it.

[mightyaleksey]

I'll link it to that Issue css-modules/css-modules-loader-core#21

[mightyaleksey]

Thank you, @joeybaker
I have published it in 1.0.5

[DenisIzmaylov]

Thank you