Switch .travis.yml to using Travis's VM infrastructure.

My request to whitelist the install of atftp seems to be lost in issue tracker
limbo, as are all other whitelisting requests for the last few months.

.travis.yml

@@ -6,10 +6,9 @@ go:
   - tip
 os:
   - linux
-addons:
-  apt:
-    packages:
-      - atftp
+before_install:
+  - sudo apt-get -qq update
+  - sudo apt-get install -y atftp
 install:
   - go get github.com/Masterminds/glide
   - go get github.com/golang/lint/golint