start running the tests on CI again

Now that they're both faster and more reliable than before the refactor.

On my laptop, 'go test' now consistently takes ~10s, and I haven't found
any flakes in the past couple of days.

.travis.yml

@@ -3,5 +3,9 @@ language: go
 go:
   - 1.12.x
 
+addons:
+  apt:
+    chrome: stable
+
 script:
-  - true
+  - go test -v ./...