have CI run using elm 0.19.1

Thanks to Kalabibishkis on slack for this syntax.
We have to pick the latest version of elm-format even though
we do not use it to avoid an install error on travis.

Author: harrysarson

.travis.yml

@@ -1,4 +1,11 @@
 language: elm
 
+elm:
+  - latest
+
+elm_test: latest
+
+elm_format: latest
+
 script:
   - bash tests/run-tests.sh