add go.mod files, set GO111MODULE=on in Travis build

Author: dmitris

.travis.yml

@@ -15,7 +15,7 @@ before_install:
 install:
   - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
 script:
-  - go get -t -v ./...
+  - GO111MODULE=on go get -t -v ./...
   - diff -u <(echo -n) <(gofmt -d -s .)
-  - go tool vet .
-  - go test -v -race ./...
+  - GO111MODULE=on go tool vet .
+  - GO111MODULE=on go test -v -race ./...

go.mod

@@ -0,0 +1,8 @@
+module sourcegraph.com/sourcegraph/go-diff
+
+require (
+	github.com/gogo/protobuf v1.1.1
+	github.com/shurcooL/go v0.0.0-20180423040247-9e1955d9fb6e // indirect
+	github.com/shurcooL/go-goon v0.0.0-20170922171312-37c2f522c041
+	sourcegraph.com/sqs/pbtypes v0.0.0-20180604144634-d3ebe8f20ae4
+)

go.sum

@@ -0,0 +1,8 @@
+github.com/gogo/protobuf v1.1.1 h1:72R+M5VuhED/KujmZVcIquuo8mBgX4oVda//DQb3PXo=
+github.com/gogo/protobuf v1.1.1/go.mod h1:r8qH/GZQm5c6nD/R0oafs1akxWv10x8SbQlK7atdtwQ=
+github.com/shurcooL/go v0.0.0-20180423040247-9e1955d9fb6e h1:MZM7FHLqUHYI0Y/mQAt3d2aYa0SiNms/hFqC9qJYolM=
+github.com/shurcooL/go v0.0.0-20180423040247-9e1955d9fb6e/go.mod h1:TDJrrUr11Vxrven61rcy3hJMUqaf/CLWYhHNPmT14Lk=
+github.com/shurcooL/go-goon v0.0.0-20170922171312-37c2f522c041 h1:llrF3Fs4018ePo4+G/HV/uQUqEI1HMDjCeOf2V6puPc=
+github.com/shurcooL/go-goon v0.0.0-20170922171312-37c2f522c041/go.mod h1:N5mDOmsrJOB+vfqUK+7DmDyjhSLIIBnXo9lvZJj3MWQ=
+sourcegraph.com/sqs/pbtypes v0.0.0-20180604144634-d3ebe8f20ae4 h1:JPJh2pk3+X4lXAkZIk2RuE/7/FoK9maXw+TNPJhVS/c=
+sourcegraph.com/sqs/pbtypes v0.0.0-20180604144634-d3ebe8f20ae4/go.mod h1:ketZ/q3QxT9HOBeFhu6RdvsftgpsbFHBF5Cas6cDKZ0=