proto3 timestamp (#2064)

This PR changes ABCI time format from int64 (Unix seconds) to WKT (WellKnownType) google.protobuf.Timestamp.

Refs #1857

Reasons:

better precision
standard DT for proto

* update Gopkg.lock
* [makefile] remove extra grep
    - go list excludes vendor by default now
* proto3 timestamp
* [docs/abci-spec] note about serialisation format
* make time non-nullable

proxy/app_conn_test.go

@@ -142,7 +142,7 @@ func TestInfo(t *testing.T) {
 	proxy := NewAppConnTest(cli)
 	t.Log("Connected")
 
-	resInfo, err := proxy.InfoSync(types.RequestInfo{""})
+	resInfo, err := proxy.InfoSync(types.RequestInfo{Version: ""})
 	if err != nil {
 		t.Errorf("Unexpected error: %v", err)
 	}