Update peerDependencies for webpack2

[ju1ius]

Hi,
although the plugin works quite well with webpack2.1.0-beta4, it specifies a peerDependency to webpack 1, causing CI build to fail:

npm ERR! peerinvalid Peer extract-text-webpack-plugin@1.0.1 wants webpack@^1.9.11

[timbru31]

Running into the same issue, the ExtractTextPlugin works fine though.

[amio]

There is a PR #157 already, but nobody merge it yet.

[amio]

Acturally PR #147 may solve this too.

[Jessidhia]

This also complicates use with npm 2 -- installing extract-text-webpack-plugin will "update" webpack@^2.1.0-beta into one that matches the peerDependency.

[developer94404]

just a note:

using npm 4.x on my system:

1. on my system, installing using node v4.x fails, and gives:
   npm WARN peerDependencies The peer dependency webpack@^1.9.11 included from extract-text-webpack-plugin will no
   npm WARN peerDependencies longer be automatically installed to fulfill the peerDependency
   npm WARN peerDependencies in npm 3+. Your application will need to depend on it explicitly.
   npm ERR! node v4.2.3
   npm ERR! npm v2.14.7
   npm ERR! code EPEERINVALID

2. and then using node v7.x is successful, but gives:
   +-- extract-text-webpack-plugin@1.0.1
   `-- UNMET PEER DEPENDENCY webpack@2.2.0