Polyfill option is removed from babel-transform-runtime RC4, causing build to break

[balcsida]

During the manual testing of #81, building the bookmark-it extension failed because Babel 7-RC4 removed polyfill option from transform-runtime (see babel/babel#8547 for more info on that).

First of all, I would like to fix this issue, it seems that simply removing this configuration option from our side is enough.
Secondly, I will open a separate Issue, so next time npm test will try to build the bookmark-it example extension, so Travis and AppVeyor has greater chance to catch these kind of issues.