chore(deps): remove acorn

[SukkaW]

acorn is introduced in #50. However, I don't see why the acorn is required.

Seeing the related issue, still don't know why acorn should be included in dependencies.

The PR shouldn't be merged before @tomap gives the explanation, in case the PR breaks something.

[coveralls]

Coverage remained the same at 85.062% when pulling 58032bc on SukkaW:remove-acorn into af92881 on hexojs:master.

[tomap]

Hi, I'm not sure why I I introduced it. Two possible reasons:
Npm was asking for it (peerdependency) or
There was a security issue with a secondary dependency, and including it directly fixed the issue
In any case, you can remove it if you tested the removal without issue!

[SukkaW]

@tomap I have runned npm ls acorn. It appears that acorn is a dependency of eslint & rewire.

Since it is a dependency of two devDependencies, I think we could remove it without breaking normal users (Yes, maybe developers like us might be affected, but is doesn't matter much, right?).