all: update to go1.13.15, go1.14.7, and go1.15

Change-Id: I9675b6e1196afb185fab1c2d650894cf68b325e5
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/248277
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/integration_test.go b/integration_test.go
index d24d70f..980d5de 100644
--- a/integration_test.go
+++ b/integration_test.go
@@ -36,7 +36,7 @@
 	protobufVersion = "ef7cc811" // v3.12.0-rc1
 	protobufSHA256  = ""         // ignored if protobufVersion is a git hash
 
-	golangVersions = []string{"1.9.7", "1.10.8", "1.11.13", "1.12.17", "1.13.12", "1.14.4"}
+	golangVersions = []string{"1.9.7", "1.10.8", "1.11.13", "1.12.17", "1.13.15", "1.14.7", "1.15"}
 	golangLatest   = golangVersions[len(golangVersions)-1]
 
 	staticcheckVersion = "2020.1.4"