conf-common: Replace http with https for Google Font

As we are now switching to SSL for the user manual
all our linked content needs to be updated to prevent
warning messages.

source/conf-common.py

@@ -119,7 +119,7 @@ html_theme = 'mahara'
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
 # documentation.
-html_theme_options = {'googlewebfont': True, 'googlewebfont_url': 'http://fonts.googleapis.com/css?family=Open+Sans:300,300i,400,400i,700,700i,800,800i', 'googlewebfont_style': "font-family: 'Open Sans', sans-serif",}
+html_theme_options = {'googlewebfont': True, 'googlewebfont_url': 'https://fonts.googleapis.com/css?family=Open+Sans:300,300i,400,400i,700,700i,800,800i', 'googlewebfont_style': "font-family: 'Open Sans', sans-serif",}
 
 # Add any paths that contain custom themes here, relative to this directory.
 # html_theme_path = ['.']