Use view.jobIdForLog instead of showDownloadLog

The latter is not always refreshed, I don't have time to debug it more
thoroughly

assets/scripts/app/templates/repos/show/tools.hbs

@@ -37,7 +37,9 @@
         </a>
       </li>
     {{/if}}
-    {{#if view.showDownloadLog}}
+    {{!TODO: for some reason showDownloadLog, which just delegates to jobIdForLog
+             does not refresh 'if' properly, need further investigation}}
+    {{#if view.jobIdForLog}}
       <li>
         <a class="download-log" {{bindAttr href="view.plainTextLogUrl"}}>Download log</a>
       </li>

assets/scripts/lib/travis/ajax.coffee

@@ -76,7 +76,7 @@ Travis.ajax = Em.Object.create
 
     xhr.open(method, url)
 
-    if options.accepts
+    if options.accepts && !options.headers?.accept?
       accepts = []
       for key, value of options.accepts
         accepts.pushObject(value)