Updates Travis to use ruby 2.6.0 (#736)

Author: LeFnord

.travis.yml

@@ -13,7 +13,7 @@ after_success:
 rvm:
   - 2.4.5
   - 2.5.3
-  - 2.6.0-rc1
+  - 2.6.0
 env:
   - MODEL_PARSER=grape-swagger-entity
   - MODEL_PARSER=grape-swagger-representable