Downgrade acorn-jsx dependency to resolve conflicts with recent version of acorn

[iansan5653]

Buble is not compatible with acorn^7.0.0, as is correctly declared in the package.json. Normally this would be the end of it, but because acorn-jsx is compatible with acorn^6.0.0 || ^7.0.0, this causes Buble to fail if acorn^7.0.0 exists anywhere where acorn-jsx can grab it as then acorn-jsx is using 7 and Buble wants to use 6.

In other words, Buble claims to be compatible with acorn-jsx^5.0.1 but is actually not and should instead say acorn-jsx^4.1.1, which is the most recent version of acorn-jsx that does not claim compatibility with acorn^7.0.0.

[iansan5653]

This is what's causing styleguidist/react-styleguidist#1321

[iansan5653]

I tried to write a PR for this, but acorn-jsx didn't start using the new acorn extending method until 5.0, so I don't know how to make it work.

[iansan5653]

Closing because this is a bug that should actually be fixed in the acorn-jsx repo.