Add Rails 6.1 ActiveRecord support

[johan-smits]

Issue #, if available: #56

Description of changes: Support ActiveRecord 6.1

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

[anuraaga]

Thanks a lot @johan-smits! This change looks fine to me - I noticed we don't seem to have any tests for rails :( Would adding a test that uses rails 6.1 be something you can help us with? If not no worries and we'll merge this

[johan-smits]

@anuraaga I did not find any tests for ActiveRecord thats why I did not add one. If you can point me out where they are I'm happy to add a new one.

[anuraaga]

Right :) That's the same conclusion I came to when looking through this repo for the first time. So it would start from adding rails as a dev dependency in the gemspec.

You don't have to do it of course :) Our team unfortunately doesn't have anyone super familiar with rails right now so we really appreciate the help from the community here - if you could add the dependency and a test using it similar to what we have in test, it would be really awesome. If not, it's also still awesome to have this PR so no worries.

[johan-smits]

@anuraaga not sure on how to make a test for this, I'm familiar with ActiveRecord but not so much on the testing area. Since it this code

aws-xray-sdk-ruby/lib/aws-xray-sdk/facets/rails/active_record.rb

Lines 10 to 13 in d5de6c2

	DB_TYPE_MAPPING = {
	mysql2: 'MySQL',
	postgresql: 'PostgreSQL'
	}.freeze

also only works with MySQL and PG, so this should also be setup to test this change.

[anuraaga]

@johan-smits No worries let's merge this in :) Thanks!

[johan-smits]

@anuraaga thanks, when would this get released?