Updates bundler in AppVeyor

[sthulb]

Issue #, if available:

Description of changes:
Pins bundled version on AppVeyor.

AppVeyor drops out with:

ERROR:  Error installing bundler:
	The last version of bundler (>= 0) to support your Ruby & RubyGems was 1.17.3. Try installing it with `gem install bundler -v 1.17.3`
	bundler requires RubyGems version >= 3.0.0. The current RubyGems version is 2.7.8. Try 'gem update --system' to update RubyGems itself.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jfuss]

Should we just update RubyGems instead of pinning?

I don't understand ruby enough to understand which is better: @awood45?

[sthulb]

I tried updating rubygems first. It added a considerable amount of time to the build.

[jfuss]

@sthulb I see. I am ok pinning this. We don't have any min version requirement so this seems fine to pin given the build time.

[awood45]

This approach is reasonable.