Only show master branch CI status on the website.

I think not doing `?branch=master` means travis will show a badge for
the latest build, even if it is a PR. This caused the badge to sometimes
show "Build failing" even if master was passing just fine.

website/themes/uppy/layout/partials/social.ejs

@@ -6,7 +6,7 @@ allowtransparency="true" frameborder="0" scrolling="0" width="100" height="20"><
   <li><a href="https://community.transloadit.com/c/uppy"><img src="/images/community-forum-d8006a.svg" /> </a></li>
   <li class="BuildBadge">
    <span class="wrapper">
-     <a href="https://travis-ci.org/transloadit/uppy"><img src="https://travis-ci.org/transloadit/uppy.svg" alt="Build Status"></a>
+     <a href="https://travis-ci.org/transloadit/uppy"><img src="https://travis-ci.org/transloadit/uppy.svg?branch=master" alt="Build Status"></a>
    </span>
   </li>
   <li class="SauceBadge">