fix: vendor/golang.org/x/net/http2/Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU1404-OPENSSL-1049144
- https://snyk.io/vuln/SNYK-UBUNTU1404-OPENSSL-3314800
- https://snyk.io/vuln/SNYK-UBUNTU1404-SUDO-1065770
- https://snyk.io/vuln/SNYK-UBUNTU1404-SUDO-3234995
- https://snyk.io/vuln/SNYK-UBUNTU1404-SUDO-473059

vendor/golang.org/x/net/http2/Dockerfile

@@ -6,7 +6,7 @@
 # Go tests use this curl binary for integration tests.
 #
 
-FROM ubuntu:trusty
+FROM ubuntu:trusty-20190515
 
 RUN apt-get update && \
     apt-get upgrade -y && \