chore(package): update webpack to version 1.12.15
chore(package): update webpack to version 1.12.15

http://greenkeeper.io/
shubheksha/web-ext,kumar303/web-ext,kumar303/web-ext,rpl/web-ext,shubheksha/web-ext,mozilla/web-ext,rpl/web-ext,mozilla/web-ext