add deploy script which only deploys on master

.travis.yml

@@ -19,4 +19,5 @@ script:
 - bundle exec nanoc check mixed_content
 after_success:
 - chmod 600 ~/.ssh/id_rsa
-- bundle exec nanoc deploy --target public
+- chmod +x .travis/deploy.sh
+- .travis/deploy.sh

.travis/deploy.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+if [[ $TRAVIS_BRANCH == 'master' ]]
+  bundle exec nanoc deploy --target public
+fi