Fix an error for test

```
Error:
Coverband::AuthWebTest#test_renders_index_with_basic_auth:
NameError: uninitialized constant Base64
    /Users/yudai.takada/ydah/coverband/lib/coverband/reporters/web.rb:46:in `check_auth'
    /Users/yudai.takada/ydah/coverband/lib/coverband/reporters/web.rb:57:in `call'
    /Users/yudai.takada/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rack-test-2.1.0/lib/rack/test.rb:360:in `process_request'
    /Users/yudai.takada/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rack-test-2.1.0/lib/rack/test.rb:163:in `custom_request'
    /Users/yudai.takada/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rack-test-2.1.0/lib/rack/test.rb:112:in `get'
    /Users/yudai.takada/.rbenv/versions/3.2.2/lib/ruby/3.2.0/forwardable.rb:240:in `get'
    /Users/yudai.takada/ydah/coverband/test/coverband/reporters/web_test.rb:69:in `block in <class:AuthWebTest>'
```

lib/coverband/reporters/web.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "base64"
 require "coverband"
 
 begin