Get libcurl to use openssl instead of gnutls on Travis

We've moved the site to HTTPS today and now the build is failing, for example:

    *  External link http://[secure]rence.org/spa2016 failed: 301 SSL connect error

This fix is recommended on the htmlproofer wiki [1], following on from
gjtorikian/html-proofer#376.

[1]: https://github.com/gjtorikian/html-proofer/wiki/Using-HTMLProofer-From-Ruby-and-Travis

.travis.yml

@@ -21,3 +21,7 @@ deploy:
   skip_cleanup: true
   on:
     branch: master
+addons:
+  apt:
+    packages:
+    - libcurl4-openssl-dev # required to avoid SSL errors