Test on all supported OSes (#22)

smondal · sindresorhus · commit 8ec64dcb35b3 · 2019-12-31T19:33:57.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,3 +1,7 @@
+os:
+  - osx
+  - linux
+  - windows
 language: node_js
 node_js:
   - '12'
