Use Node.js v12, which is now necessary for BigInts.

sjrd · sjrd · commit 495f4b83a279 · 2019-11-18T14:09:38.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,9 @@
 dist: xenial
 language: scala
+before_install:
+  - nvm install 12.13.0
 script:
+  - node -v
   - ./run.sh $SCALA_BRANCH
 scala:
   # This is actually overridden by the run.sh script
