travis: Test with oldstable and stable (#39)

Use the most recent two minor versions of Go for testing.

Use `oldstable` and `stable` introduced in gimme v1.5.4.

Co-authored-by: Abhinav Gupta <mail@abhinavg.net>

Author: sagikazarmark

.travis.yml

@@ -4,17 +4,11 @@ go_import_path: go.uber.org/multierr
 
 env:
   global:
-    - GO15VENDOREXPERIMENT=1
     - GO111MODULE=on
 
 go:
-  - 1.11.x
-  - 1.12.x
-  - 1.13.x
-
-cache:
-  directories:
-    - vendor
+  - oldstable
+  - stable
 
 before_install:
 - go version