Patch fake server without clock

[mantoni]

Purpose (TL;DR) - mandatory

Use nise.fakeServer as the default sandbox serverPrototype to fix #1534.

Apparently the Chrome installation has changed and it is now necessary to pass the --allow-chrome-as-root flag on Travis like we already do on CircleCI.

Background (Problem in detail) - optional

Creating a sandbox with fake server installed fake timers on the first XHR request. Both ways to install fake servers are affected:

sandbox = sinon.createSandbox({
  useFakeServer: true
});

and

sandbox = sinon.createSandbox();
sandbox.useFakeServer();

Solution - optional

Setting nise.fakeServer as the default serverPrototype on the sandbox fixes the issue.

How to verify - mandatory

The new tests ensure the correct serverPrototype is used and the fake server returned by useFakeServer() is not an instance of fakeServerWithClock anymore.

Checklist for author

• npm run lint passes
• References to standard library functions are cached.