Removed deprecated method .development()

[shyjal]

[webpack-isomorphic-tools] [error] .development() method is now deprecated (for server-side instance only, not for webpack plugin instance) and has no effect. Set up a proper process.env.NODE_ENV variable instead: it should be "production" for production, otherwise it assumes development. The currently used mode is: production. `process.env.NODE_ENV is: production

Fixed the above error while running npm run build:static