dashboard: images should be fetched from HTTPS
dashboard: images should be fetched from HTTPS

Revision 184f9219148f (from review 152790043) introduced a new image,
linux-x86-clang, and the url is missing the `s` for TLS.

LGTM=adg
R=bradfitz, golang-codereviews, adg
CC=golang-codereviews
https://codereview.appspot.com/156480043

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools