Add Travis CI including a check for gofmt -s

Attempt building with both go1 and the currrent go release, to ensure
compatibility.

Signed-off-by: Robert Breker <robert.breker@citrix.com>

.travis.yml

@@ -0,0 +1,7 @@
+language: go
+go:
+  # Test with the first and the latest go release - to ensure compatibility
+  - 1
+  - release
+script:
+  - gofmtresult=$(gofmt -s -l .); if [[ -n $gofmtresult ]]; then echo -e "Please run \"gofmt -s -w .\" before committing for the below:\n$gofmtresult"; false; fi

README.md

@@ -1,3 +1,5 @@
+[![Build Status](https://travis-ci.org/rdobson/packer-builder-xenserver?branch=master)](https://travis-ci.org/rdobson/packer-builder-xenserver)
+
 # XenServer packer.io builder
 
 This builder plugin extends packer.io to support building images for XenServer.