Looks like npm bug, since webpack-dev-server@2.1.0-beta.11 requires webpack@^2.1.0-beta.26 but npm failed to install it.
The easiest way to avoid the issue without updating too much is to change dependency in package.json to
"webpack-dev-server": "2.1.0-beta.10",
Instead of something like
"webpack-dev-server": "^2.1.0-beta.9",
“^” char before version says “compatible with”. Removing it sticks to the version exactly.
Don’t forget to run npm install or npm update afterwards.