Only deploy from the Elasticsearch build job

Fixes #1099, or at least works around it, though a better solution would
be to rewrite our deployment system using Travis build stages or Heroku
build pipelines.

The Elasticsearch job was picked because it's the one most likely to
fail, and I can't work out how to deploy only if all three jobs pass.

.travis.yml

@@ -56,6 +56,7 @@ deploy:
     secure: "WrQxf0fEKkCdXrjcejurobOnNNz3he4dDwjBbToXbQTQNDObPp7NetJrLsfM8FiUFEeOuvhIHHiDQtMvY720zGGAGxDptvgFS+0QHCUqoTRZA/yFfUmHlG2jROXTzk5uVK0AE4k6Ion5kX8+mM0EnMT/7u+MTFiukrJctSiEXfg="
   on:
     repo: Growstuff/growstuff
+    condition: "$RSPEC_TAG = elasticsearch"
   app:
     dev: growstuff-staging
     master: growstuff-prod