CI: Travis YAML, avoid float-to-string conversion

In some places, a float of 3.0 can be turned into the string "3".

.travis.yml

@@ -13,7 +13,7 @@ rvm:
   - 2.5
   - 2.6
   - 2.7
-  - 3.0
+  - "3.0"
 matrix:
   include:
     - rvm: 2.4