(docs/fix): only report build status for master branch

- shouldn't show error, failed, etc when a different branch fails as
  they're branches and often have some degree of WIP to them
  - would also show error for PRs, etc, and the point of that is
    feedback; shouldn't make the repo look buggy in the process

Author: agilgur5

README.md

@@ -11,7 +11,7 @@
 [![dw](https://img.shields.io/npm/dw/mst-persist.svg)](https://npmjs.org/package/mst-persist)
 <br><!-- status / activity -->
 [![typings](https://img.shields.io/npm/types/mst-persist.svg)](https://github.com/agilgur5/mst-persist/blob/master/src/index.ts)
-[![build status](https://img.shields.io/travis/agilgur5/mst-persist.svg)](https://travis-ci.org/agilgur5/mst-persist)
+[![build status](https://img.shields.io/travis/agilgur5/mst-persist/master.svg)](https://travis-ci.org/agilgur5/mst-persist)
 <br>
 [![NPM](https://nodei.co/npm/mst-persist.png?downloads=true&downloadRank=true&stars=true)](https://npmjs.org/package/mst-persist)