all: fix Travis-CI

The v1 implementation has support for some legacy proto1 features.
However, these features are not enabled by default in v2.
Thus, specify the proto1_legacy flag.

Change-Id: Ieb160188f9d50385076b8e9f08ccc4cebb02bae1
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/186619
Reviewed-by: Herbie Ong <herbie@google.com>

.travis.yml

@@ -7,10 +7,10 @@ before_install:
 matrix:
   include:
     - go: 1.9.x
-      script: go test -v ./...
+      script: go test -tags proto1_legacy -v ./...
     - go: 1.10.x
-      script: go test -v ./...
+      script: go test -tags proto1_legacy -v ./...
     - go: 1.11.x
-      script: go test -v ./...
+      script: go test -tags proto1_legacy -v ./...
     - go: 1.12.x
       script: ./test.bash