Changed Webpack 2 peer dependency to 2.2 rc ver

[harounb]

Did this to resolve the unmet peer dependency error.
If you would like to retain support for 2.1 let me know and I'll amend.
I assumed you wouldn't as you're only supporting one version of webpack v1

Edit:
Thinking about this a little more could someone explain to me why the current package.json doesn't work fine?
the jump from 2.1 to 2.2 is a minor release so surely ^2.1 should cover that.
I'm quite confused even though it does appear to have worked.
If what I have done isn't suitable please let me know and I shall close.

Thanks

[kmontag]

@harounb I believe between pre-release versions, ^ treats minor version bumps as it would normally treat major ones.

[mattlewis92]

I think that ^2 || ^2.2.0-rc.0 || ^2.1.0-beta || ^1.12.6 might be even better, as this is the convention the webpack loaders / plugins on the webpack org seem to be doing.

[graingert]

@jtangelder can we get this merged?

[jtangelder]

@mattlewis92 example of which loaders do this?

[graingert]

@jtangelder I'd just copy babel-loader: https://github.com/babel/babel-loader/blob/master/package.json#L17

[mattlewis92]

@jtangelder there's a couple examples here as well:
https://github.com/webpack/karma-webpack/blob/master/package.json#L73
https://github.com/webpack/webpack-dev-server/blob/master/package.json#L7

[jtangelder]

Sorry @harounb, I'm having some git issues on my machine, wasn't able to enrich this PR. Still thanks!

[harounb]

@jtangelder No problem and great job :)
Apologies for vanishing after submitting this.