* Put current version of babel-types in the root
Previously, the code that supports the oldest version of Typescript was in the root: 3.1 and below. Newer versions were in ts*/ subdirectories.
This PR puts the newest version in the root, and older versions -- before 3.7 in this case -- in ts*/ subdirectories. This is possible because all supported versions of the Typescript now understand the typesVersions property in package.json.
This PR needs a new version of DefinitelyTyped-tools and dtslint, which I will ship soon. It also needs to be brought up to date with master.
* delete ts3.7/
* use correct relative reference for test
