Remove superfluous quotes from config path

[bluecmd]

Using -config="path" will make the application try to open paths with the quotes included which is not what is expected.

main.go:89: ERROR: failed to load config file "/etc/oauth2_proxy.cfg" - open "/etc/oauth2_proxy.cfg": no such file or directory

[rremer]

@bluecmd you may want to fetch upstream and repush, I suspect your build will be fixed by #249