README.md: add Travis CI badge

Cosimo Lupo · Cosimo Lupo · commit ec316a4c6183 · 2016-10-18T13:13:19.000+01:00
It happened again, sorry. I've tagged the previous commit but wrote the two magic words and Travis didn't run the build :(
Apologies. Let's try again.
diff --git a/README.md b/README.md
@@ -1,3 +1,4 @@
+[![Build Status](https://api.travis-ci.org/typemytype/booleanOperations.svg)](https://travis-ci.org/typemytype/booleanOperations)
 [![PyPI](https://img.shields.io/pypi/v/booleanOperations.svg)](https://pypi.org/project/booleanOperations/)
 ![Python Versions](https://img.shields.io/badge/python-2.7%2C%203.4%2C%203.5-blue.svg)
 

Original file line number	Diff line number	Diff line change
`@@ -1,3 +1,4 @@`
	`1`	`+[![Build Status](https://api.travis-ci.org/typemytype/booleanOperations.svg)](https://travis-ci.org/typemytype/booleanOperations)`
`1`	`2`	`[![PyPI](https://img.shields.io/pypi/v/booleanOperations.svg)](https://pypi.org/project/booleanOperations/)`
`2`	`3`	`![Python Versions](https://img.shields.io/badge/python-2.7%2C%203.4%2C%203.5-blue.svg)`
`3`	`4`