Change version of async dep to ~2.6.3 for IE compatibility.

[mattwelke]

@Atinux
We've been sorting out the Node and IE compatibility issue in #77. By now, we've reached consensus that version 1.6.9 of schema-inspector accidentally had breaking changes because of its udnerlying async dependency.

I think moving from the version 3 branch to version 2 branch of async is a great next step because it resolves the breaking changes issue and async's maintainers have committed to maintaining their version 2 branch with IE compatibility and it will receive patches for CVEs until further notice: caolan/async#1661 (comment)

This PR changes us to 2.6.3 of async, and should keep us on the version 2 branch because of specifiying ~2.6.3. It doesn't use ^2.6.3, because we don't need new async features, just patches for CVEs. The PR does not increment the version of schema-inspector, because I notice you do the version increment in separate commits, so I'm respecting that pattern.

[jcdesousa]

@mattwelke great work opening the review 👏

@Atinux can we get this approved and release a hotfix 🙏

[jcdesousa]

@Atinux ☝️ :prettyplease:

[jcdesousa]

Can someone merge this?

[Atinux]

Released, sorry for delay, my wife had our newborn child last week

[mattwelke]

No problem. Thanks for releasing this today. And congratulations!

[Atinux]

@mattwelke would you be interested to help me maintain this project?

[mattwelke]

@Atinux Yes! We'll be using this library at my work for the foreseeable future and it seems to have a narrow, stable focus that will be easy to maintain without too many feature requests.

@jcdesousa may be interested too.

[Atinux]

Invited!

Could you also tell me you npm username?

[mattwelke]

@Atinux NPM username is the same as my GitHub username: mattwelke. I recently remade my NPM account since I used to use one that was shared between personal stuff and my work stuff. So this new one is completely separate from my work stuff and should be safe to use for open source. Let me know if you'd like me to create a dedicated NPM account though. I've never maintained an open source project so I'm not familiar with best practices for NPM accounts for them yet.

[Atinux]

No worries, just added you :)