travis.yml: update tested versions of Go (#1211)

While this module is deprecated, it is still important to ensure
that it continues to work with higher versions of Go.
Update travis.yml to test up to Go1.15.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 720ef9d..0b51666 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -13,4 +13,10 @@
     - go: 1.11.x
       script: go test -tags protolegacy -v ./...
     - go: 1.12.x
+      script: go test -tags protolegacy -v ./...
+    - go: 1.13.x
+      script: go test -tags protolegacy -v ./...
+    - go: 1.14.x
+      script: go test -tags protolegacy -v ./...
+    - go: 1.15.x
       script: ./test.bash