Update README.md

Author: Keats

README.md

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 # mmap-bitvec #
 
-[![CircleCI](https://circleci.com/gh/onecodex/mmap-bitvec.svg?style=svg&circle-token=dcb1850cbbec3e55d28cec4cb5082bb30199cf97)](https://circleci.com/gh/onecodex/mmap-bitvec)
+![ci](https://github.com/onecodex/mmap-bitvec/workflows/ci/badge.svg)
 
 mmap-bitvec is a library for working with mmap-backed bit-vectors and some simple
 data structures derived from bit-vectors.