travis: no windows. A known failing test is worse than no test.

https://travis-ci.org/npm/node-tar/jobs/443305190

Author: isaacs

.travis.yml

@@ -1,14 +1,11 @@
 language: node_js
+sudo: false
 
 node_js:
   - 10
   - 8
   - 6
 
-os:
-  - linux
-  - windows
-
 cache:
   directories:
     - $HOME/.npm