Load Google css over HTTPS to prevent loading error in HTTPS-only instances.

The change is required to prevent the following error (From Chrome's console):

> Mixed Content: The page at 'https://redmineinstance' was loaded over HTTPS, but requested an insecure stylesheet 'http://fonts.googleapis.com/css?family=Lato:300,400,700'. This request has been blocked; the content must be served over HTTPS.