travis.yml: s/proto1_legacy/protolegacy/

Fix tag used to enable MessageSet support.

Change-Id: Iba7d92038beb28f563923c96e477656705b34cbd
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/190980
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

.travis.yml

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