CI: Use bash not sh for .travis/deploy.sh (#198)

We use bashisms in the `deploy.sh` script and need to invoke with with
`bash` not `sh`, despite the shebang at the top of the deploy script.

Author: cpu

.travis.yml

@@ -39,7 +39,7 @@ script:
 
 deploy:
   - provider: script
-    script: sh .travis/publish.sh
+    script: bash .travis/publish.sh
     skip_cleanup: true
     on:
-      repo: letsencrypt/pebble
+      repo: letsencrypt/pebble