refactor(package): use Object.assign instead of object-assign dependency

[yoshinorin]

Proposal

Use Object.assign instead of object-assign package

Reason

Node.js 4 and up can be use Object.assign() instead of object-assign npm package.
This repository is already set node engine higher than 6.9.0 .
So, it can use Object.assign() .

[coveralls]

Coverage remained the same at 100.0% when pulling faa9485 on YoshinoriN:delete-object-assign-dependency into 71e15be on hexojs:master.