chore: pin base64 (#340)

# Description

<!-- Please provide a general summary of your PR changes and link any
related issues or other pull requests. -->

# Testing

<!-- 
Please provide details on how you tested this code. See below.

- All pull requests must be tested (unit tests where possible with
accompanying cassettes, or provide a screenshot of end-to-end testing
when unit tests are not possible)
- New features must get a new unit test
- Bug fixes/refactors must re-record existing cassettes 
-->

# Pull Request Type

Please select the option(s) that are relevant to this PR.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] Improvement (fixing a typo, updating readme, renaming a variable
name, etc)

Author: Justintime50

easypost.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ['lib']
   spec.required_ruby_version = '>= 2.7'
 
-  spec.add_dependency 'base64'
+  spec.add_dependency 'base64', '~> 0.3'
 
   spec.add_development_dependency 'abbrev', '~> 0.1'
   spec.add_development_dependency 'benchmark', '~> 0.4'