Braintree update gem version to 4.5

[Heavyblade]

Summary:

This change updates Braintree gem version to the latest version 4.5 and sets the minimum Ruby version to 2.6 as a consequence.

Test Execution:

Unit Finished in 26.188459 seconds. 5067 tests, 75099 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed

Remote Finished in 0.903732 seconds. 86 tests, 194 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed

Rubocop:

Rubocop 728 files inspected, no offenses detected

[curiousepic]

Actually since we're removing support for a ruby version, let's coordinate this change with Shopify.