Upgrade to ast-types v0.4.1 and update tests to use new features.

Key new features: Path.prototype.unshift (and friends: shift, push, pop,
insertAt, insertBefore, insertAfter) and types.visit (which replaces
types.traverse).
Merge pull request #2 from benjamn/upgrade-ast-types

Upgrade to ast-types v0.4.1 and update tests to use new features.