Fix 'make lint' not running megacheck inside travis (#821)

After running gometalinter in debug mode, it was found megacheck was being killed by travis due to reaching its memory limits. For more information, see this comment.

Run "Lint & Vendor Check" using a fully virtualized environment instead of a container-based one.

.travis.yml

@@ -12,6 +12,7 @@ install:
 jobs:
   include:
   - stage: Lint & Vendor Check
+    sudo: required
     script:
       - make lint
       - make vendor-check