chore(deps): Migrate to sprintf-js

[spencerwilson-optimizely]

Summary

• Migrate from sprintf to sprintf-js

The former is deprecated, and as a result when customers install the package they see

$ npm i @optimizely/optimizely-sdk
npm WARN deprecated sprintf@0.1.5: The sprintf package is deprecated in favor of sprintf-js.

This migration resolves that, so now it's a silent, warning-less install.

Bundlephobia: old, new. 3.0 to 3.2 kB minified.

Test plan

Exhaustiveness:

$ ag 'require.*sprintf('\''|")\)' lib/ ; echo $?
1

Also, CI.

[coveralls]

Coverage remained the same at 97.11% when pulling 5d5adae on sw/upgrade-sprintf into a72229a on master.

[tylerbrandt]

I doubt we are stubbing sprintf so if the tests pass should be fine.