Update base64 requirement from ~> 0.2.0 to ~> 0.3.0 (#555)

Updates the requirements on [base64](https://github.com/ruby/base64) to permit the latest version.
- [Release notes](https://github.com/ruby/base64/releases)
- [Commits](ruby/base64@v0.2.0...v0.3.0)

---
updated-dependencies:
- dependency-name: base64
  dependency-version: 0.3.0
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile

@@ -10,7 +10,7 @@ gem "minitest", "5.15.0"
 gem "rake-compiler", "~> 1.3.0" # Small bug in 1.2.4 that breaks Ruby 2.5
 gem "rake-compiler-dock", "1.9.1" # This should match the versions used in docker/Dockerfile.*
 gem "racc", "~> 1.7"
-gem "base64", "~> 0.2.0"
+gem "base64", "~> 0.3.0"
 gem "yard"
 gem "mutex_m"