chore: Make support for acorn 8 explicit

[sverweij]

Description

acorn-jsx works fine with acorn 8, but acorn 8 compatibility isn't listed in its peer dependencies yet, so package management tools complain on install.

This PR makes the fact acorn-jsx works OK with acorn 8 explicit. Fixes #118

Changes

• adds acorn 8 to the peer dependencies (& ups the devDependency to ^8.0.1).
• updates the .travis.yml ci config to also check against acorn 8 (and to run on node 14)
• in the tests explicitly passes the ecmaVersion (as 11) in the options in cases it wasn't set explicitly; the acorn 8.0.0 release notes mention this as the only breaking change. Setting it explicitly (1) prevents a warning in the test output and (2) likely prevents the tests from failing in the future.

Test plan

• automated non-regression tests pass
• green CI

[RReverser]

Looks great, thanks!

[RReverser]

Published as 5.3.0.