Use shield from CircleCI instead, for consistency

README.md

@@ -3,7 +3,7 @@
     src="header.png"
     width="800" height="255" border="0" alt="gps">
 <br>
-<a href="https://circleci.com/gh/sdboyer/gps"><img src="https://circleci.com/gh/sdboyer/gps.svg?style=svg" alt="Build Status"></a>
+<a href="https://circleci.com/gh/sdboyer/gps"><img src="https://circleci.com/gh/sdboyer/gps.svg?style=shield" alt="Build Status"></a>
 <a href="https://ci.appveyor.com/project/sdboyer/gps"><img src="https://ci.appveyor.com/api/projects/status/github/sdboyer/gps?svg=true&branch=master&passingText=Windows%20-%20OK&failingText=Windows%20-%20failed&pendingText=Windows%20-%20pending" alt="Windows Build Status"></a>
 <a href="https://goreportcard.com/report/github.com/sdboyer/gps"><img src="https://goreportcard.com/badge/github.com/sdboyer/gps" alt="Build Status"></a>
 <a href="https://codecov.io/gh/sdboyer/gps"><img src="https://codecov.io/gh/sdboyer/gps/branch/master/graph/badge.svg" alt="Codecov" /></a>