Relax the version constraint to support simplecov 0.22

[knu]

simplecov 0.22 has been released and this gem needs to support it.

Currently, updating simplecov 0.22 requires codecov to go back to 0.2.12 which happened to have no upper bound in the version constraint. Here's a hunk of Gemfile.lock included in the PR I received from dependabot.

-    codecov (0.6.0)
-      simplecov (>= 0.15, < 0.22)
+    codecov (0.2.12)
+      json
+      simplecov

Then codecov 0.2.12 failed to upload coverage data with the configuration that works with 0.6.0. (Bad URI error)

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[AlexWayfer]

Although I'm not against, I believe this repo is deprecated: https://github.com/codecov/codecov-ruby/commit/5c516a799bbe6fb351df86c237a3289e7cead0fe

And we should use shell uploader: https://docs.codecov.com/docs/codecov-uploader