[CI] Peg CircleCI badge link to master branch

[skip ci]

README.md

@@ -1,6 +1,6 @@
 [![Travis Build Status](https://travis-ci.org/harfbuzz/harfbuzz.svg?branch=master)](https://travis-ci.org/harfbuzz/harfbuzz)
 [![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/0t0flrxpstj9lb9w?svg=true&branch=master)](https://ci.appveyor.com/project/harfbuzz/harfbuzz)
-[![CircleCI Build Status](https://circleci.com/gh/harfbuzz/harfbuzz/tree/master.svg?style=svg)](https://circleci.com/gh/harfbuzz/harfbuzz)
+[![CircleCI Build Status](https://circleci.com/gh/harfbuzz/harfbuzz/tree/master.svg?style=svg)](https://circleci.com/gh/harfbuzz/harfbuzz/tree/master)
 [![Coverity Code Health](https://img.shields.io/coverity/scan/5450.svg)](https://scan.coverity.com/projects/behdad-harfbuzz)
 [![Codacy Code Health](https://api.codacy.com/project/badge/Grade/f17f1708783c447488bc8dd317150eaa)](https://app.codacy.com/app/behdad/harfbuzz)
 [![Codecov Code Coverage](https://codecov.io/gh/harfbuzz/harfbuzz/branch/master/graph/badge.svg)](https://codecov.io/gh/harfbuzz/harfbuzz)