Update to azure-storage 2.8.3 to fix vulnerabilities found by npm audit

[benvium]

Steps:

• install npm 6+
• run npm audit

found 5 moderate severity vulnerabilities in 105 scanned packages

This PR updates azure-storage to 2.8.3.

When you run npm audit you now get:

found 0 vulnerabilities in 82 scanned packages.

We've been using 2.8.3 for a little while now, and haven't noticed any issue with the library.