internal: fix version by using dashes instead of plusses

https://semver.org/

This commit will be tagged v0.37.3-alpha.

Change-Id: I5b9fe28bf7e8683c1cff5211d66669acd7941708
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/39690
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>
diff --git a/go.mod b/go.mod
index 871c048..332483f 100644
--- a/go.mod
+++ b/go.mod
@@ -9,7 +9,7 @@
 	github.com/google/pprof v0.0.0-20181206194817-3ea8567a2e57
 	github.com/googleapis/gax-go/v2 v2.0.4
 	github.com/jstemmer/go-junit-report v0.0.0-20190106144839-af01ea7f8024
-	go.opencensus.io v0.20.1+alpha
+	go.opencensus.io v0.20.1-alpha
 	golang.org/x/exp v0.0.0-20190121172915-509febef88a4
 	golang.org/x/lint v0.0.0-20190301231843-5614ed5bae6f
 	golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421
@@ -17,7 +17,7 @@
 	golang.org/x/text v0.3.1-0.20180807135948-17ff2d5776d2
 	golang.org/x/time v0.0.0-20181108054448-85acf8d2951c
 	golang.org/x/tools v0.0.0-20190312170243-e65039ee4138
-	google.golang.org/api v0.3.1+alpha
+	google.golang.org/api v0.3.1-alpha
 	google.golang.org/genproto v0.0.0-20190404172233-64821d5d2107
 	google.golang.org/grpc v1.19.0
 	honnef.co/go/tools v0.0.0-20190106161140-3f1c8253044a