Code for the Promise fake throws on IE11

[muodov]

Describe the bug
After the recent change, IE11 throws an exception (Promise is not defined) in this line

To Reproduce
Steps to reproduce the behavior:

1. User karma with karma-sinon
2. Run any test with sinon, and see error

Expected behavior
it should gracefully ignore that code piece if Promise is not defined in self.

Context (please complete the following information):

• Library version: 9.0.9
• Other libraries you are using: karma, karma-sinon

Additional context
I know. IE11 is not exactly a modern browser, but some companies like ours have to support them for their clients. And it sounds like the fix in sinon will be tiny

[mantoni]

I agree. The tests are already checking for promise support and the code should do that as well.

@muodov Would you care to provide a patch?

[muodov]

@mantoni sure. It's quite a busy time in my life, so give me a couple of weeks, Ill try to get to it.
If someone else needs this fix sooner, feel free to do it without waiting for me.

[muodov]

@mantoni I've created a tiny PR with a fix. Would you mind to take a look?