Add Node.js 13 to CI test matrix
diff --git a/.travis.yml b/.travis.yml
index 4996e78..fd27504 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,7 @@
 node_js:
   - "10"
   - "12"
+  - "13"
   - "14"
 
 # Travis is pretty behind the curve on their Yarn support...