all: fix Travis-CI
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 <86f376bf42b6daa37c768c730acd318ab6855de4@google.com>

golang/protobuf,golang/protobuf