Update CI badge in README (#55)

Author: odow

README.md

@@ -1,6 +1,6 @@
 # PiecewiseLinearOpt
 
-[![Build Status](https://github.com/jump-dev/PiecewiseLinearOpt.jl/workflows/CI/badge.svg?branch=master)](https://github.com/jump-dev/PiecewiseLinearOpt.jl/actions?query=workflow%3ACI)
+[![Build Status](https://github.com/jump-dev/PiecewiseLinearOpt.jl/workflows/CI/badge.svg)](https://github.com/jump-dev/PiecewiseLinearOpt.jl/actions?query=workflow%3ACI)
 [![codecov](https://codecov.io/gh/jump-dev/PiecewiseLinearOpt.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/jump-dev/PiecewiseLinearOpt.jl)
 
 A package for modeling optimization problems containing piecewise linear