Replace backslash in requirePath to slash

[chenvan]

It will cause error when there is a backslash in page.requirePath

[KyleAMathews]

@chenvan hey I'm not sure this is the right way to fix this — perhaps the best place to start would be if you could add a PR with a new test that's failing on this issue. I'd like to poke at the issue a bit. You'll want to add a site fixture like those in https://github.com/gatsbyjs/gatsby/tree/master/test/fixtures

[chenvan]

@KyleAMathews Ok, but I need time to figure out, I never do that before

[KyleAMathews]

@chenvan this series of free Egghead lessons by @kentcdodds might help!

https://medium.com/@kentcdodds/introducing-how-to-contribute-to-open-source-be67917eb704#.l9kokzc0j

Feel free to ask questions. Don't feel bad about adding an incomplete Pull Request here. Just get something going and we can help you through the rest of the process.

Thanks!