Add TravisCI build status badge to README.md.

README.md

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 go-spew
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+[![Build Status](https://travis-ci.org/davecgh/go-spew.png)]
+(https://travis-ci.org/davecgh/go-spew)
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 Go-spew implements a deep pretty printer for Go data structures to aid in
 debugging.  A comprehensive suite of tests with 100% test coverage is provided
 to ensure proper functionality.  See `test_coverage.txt` for the gocov coverage