Add Travis CI badge to README

https://travis-ci.org/opencontainers/ocitools

Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

Backported to v1.0.0.rc1 from d0ad8ca opencontainers#170 (cherry-pick applied
cleanly).

Signed-off-by: W. Trevor King <wking@tremily.us>

README.md

@@ -1,4 +1,4 @@
-# ocitools
+# ocitools [![Build Status](https://travis-ci.org/opencontainers/ocitools.svg?branch=master)](https://travis-ci.org/opencontainers/ocitools)
 
 ocitools is a collection of tools for working with the [OCI runtime specification][runtime-spec].