Fixup CI badge to point to GH workflow.
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PEX
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-.. image:: https://travis-ci.org/pantsbuild/pex.svg?branch=master
+.. image:: https://github.com/pantsbuild/pex/workflows/CI/badge.svg?branch=master
:target: https://travis-ci.org/pantsbuild/pex
.. image:: https://img.shields.io/pypi/l/pex.svg
:target: https://pypi.org/project/pex/
