refactor: use gravatar from hexo-util

[curbengh]

gravatar() is available on hexo-util 1.2.0.

[coveralls]

Coverage remained the same at 100.0% when pulling d7933aa on curbengh:gravatar into d66f225 on hexojs:master.

[tomap]

I believe we should move the gravatar dependency to hexo-util and not introduce a dependency with hexo in a plugin

[curbengh]

In addition to gravatar, I want to move url_for() and relative_url() to hexo-util. Though we might end up moving all helpers to hexo-util... 😅 (PS: I don't mind moving all helpers)

[tomap]

Let's start with gravatar :)

hexojs/hexo-util#81

[curbengh]

Let's start with gravatar :)

hexojs/hexo-util#81

refactored