Fix error resolving output path

...
Error: Invalid path src/main/webapp/js/alm/builds/pages/
    at pathToArray (/Users/kharriger/projects/alm/alm-webapp/node_modules/webpack-dev-server/node_modules/webpack-dev-middleware/node_modules/memory-fs/lib/MemoryFileSystem.js:23:38)
    at MemoryFileSystem.mkdirpSync (/Users/kharriger/projects/alm/alm-webapp/node_modules/webpack-dev-server/node_modules/webpack-dev-middleware/node_modules/memory-fs/lib/MemoryFileSystem.js:107:13)
    at MemoryFileSystem.(anonymous function) [as mkdirp] (/Users/kharriger/projects/alm/alm-webapp/node_modules/webpack-dev-server/node_modules/webpack-dev-middleware/node_modules/memory-fs/lib/MemoryFileSystem.js:187:34)
    at Tapable.<anonymous> (/Users/kharriger/projects/alm/alm-webapp/node_modules/webpack/lib/Compiler.js
A default value is merged into the provided options on line 30, so shouldn't be necessary, but doesn't hurt to be more explicit about it. 
 https://github.com/kurtharriger/grunt-webpack/blob/f9380fc41039d80ca94307e5e8104d6ea5fae819/tasks/webpack-dev-server.js#L30