Update README.md

travis build image now points to master branch

Author: tk3369

README.md

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 # SASLib.jl
 
-[![Build Status](https://travis-ci.org/tk3369/SASLib.jl.svg)](https://travis-ci.org/tk3369/SASLib.jl)
+[![Build Status](https://travis-ci.org/tk3369/SASLib.jl.svg?branch=master)](https://travis-ci.org/tk3369/SASLib.jl)
 [![codecov.io](http://codecov.io/github/tk3369/SASLib.jl/coverage.svg?branch=master)](http://codecov.io/github/tk3369/SASLib.jl?branch=master)
 
 This is a port of Pandas' read_sas function.