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://golang.org/cl/156480043

golang/build,golang/build,evandbrown/build,proppy/build,evandbrown/build,evandbrown/build,golang/build,proppy/build,proppy/build,golang/build,proppy/build