Add go.mod file (#877)

So, users can test if there are any problems
during 1.11's experimental cycle.

Author: rakyll

go.mod

@@ -0,0 +1,20 @@
+module go.opencensus.io
+
+require (
+	git.apache.org/thrift.git v0.0.0-20180807212849-6e67faa92827
+	github.com/beorn7/perks v0.0.0-20180321164747-3a771d992973
+	github.com/golang/protobuf v1.2.0
+	github.com/matttproud/golang_protobuf_extensions v1.0.1
+	github.com/openzipkin/zipkin-go v0.1.1
+	github.com/prometheus/client_golang v0.8.0
+	github.com/prometheus/client_model v0.0.0-20180712105110-5c3871d89910
+	github.com/prometheus/common v0.0.0-20180801064454-c7de2306084e
+	github.com/prometheus/procfs v0.0.0-20180725123919-05ee40e3a273
+	golang.org/x/net v0.0.0-20180821023952-922f4815f713
+	golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f
+	golang.org/x/sys v0.0.0-20180821140842-3b58ed4ad339
+	golang.org/x/text v0.3.0
+	google.golang.org/api v0.0.0-20180818000503-e21acd801f91
+	google.golang.org/genproto v0.0.0-20180817151627-c66870c02cf8
+	google.golang.org/grpc v1.14.0
+)