Add missing os key in travis config

without this, the travis linter tells us the following warning:

```
root: missing os, using the default linux
```

diff --git a/.travis.yml b/.travis.yml
index cc77ff940ea2e0c67b00e88b0693b1e00bb625ee..b7185f710499b34cb2577bbf0a44eb3b397d2d10 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,5 @@
 ---
+os: linux
 dist: bionic
 language: ruby
 cache: bundler