Define regeneratorRuntime globally as a side-effect, again. (#369)

* Define regeneratorRuntime globally as a side-effect, again.

In #353, I simplified the implementation of regenerator-runtime by
converting runtime.js to a normal CommonJS module. No more global object
detection, and no more sneaky capture/deletion of regeneratorRuntime.

Unfortunately, folks who use only @babel/polyfill and not
@babel/transform-runtime may have existing code that relies on a global
regeneratorRuntime variable. That code will eventually disappear
(:crossed_fingers:), but in the meantime we can continue supporting it by
simply ensuring that regeneratorRuntime is defined globally as a
side-effect of importing regenerator-runtime.

This turns out to be much easier than detecting the global object, as long
as runtime.js runs as a non-strict CommonJS module. See my code comments
for another fallback solution that should cover all remaining cases.

Background discussion:
babel/babel#7646 (comment)

Fixes #363.
Fixes #337.
Fixes #367.

* Update regenerator/package-lock.json.

* Fix Travis CI tests by running `npm i` instead of `npm ci`.

* Prevent Travis from running `npm ci` by default.

.travis.yml

@@ -1,6 +1,11 @@
 before_install:
   - npm install --global npm@latest
 
+# Prevent Travis from running `npm ci` by default, which has trouble with
+# file: dependencies.
+install:
+  - npm install
+
 language: node_js
 node_js:
   - "10"