Add Travis CI badge for master branch

Author: jimmysoda

README.md

@@ -1,6 +1,8 @@
 pdal-c: PDAL C API
 ==================
 
+[![Build Status](https://travis-ci.com/Simverge/pdal-c.svg?branch=master)](https://travis-ci.com/Simverge/pdal-c)
+
 *pdal-c* is a C API for the Point Data Abstraction Library ([PDAL](http:/github.com/PDAL/PDAL))
 and is compatible with PDAL 1.7 and later.