fix CircleCI badge

It appears that the badge was linked to a static image. Update the markdown to show the current build status instead.

Author: levlaz

README.md

@@ -1,7 +1,7 @@
 LaunchDarkly SDK for Java
 =========================
 
-![Circle CI](https://circleci.com/gh/launchdarkly/java-client.png)
+[![Circle CI](https://circleci.com/gh/launchdarkly/java-client.svg?style=shield)](https://circleci.com/gh/launchdarkly/java-client)
 [![Javadocs](http://javadoc.io/badge/com.launchdarkly/launchdarkly-client.svg)](http://javadoc.io/doc/com.launchdarkly/launchdarkly-client)
 [![FOSSA Status](https://app.fossa.io/api/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Flaunchdarkly%2Fjava-client.svg?type=shield)](https://app.fossa.io/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Flaunchdarkly%2Fjava-client?ref=badge_shield)