Parse error: syntax error, unexpected ':' in /home/content/13/4798213/html/webhero/wp-cont

Hello,

I am getting the following error on trying to network activate Pro Sites for the first time on my site.

Parse error: syntax error, unexpected ':' in /home/content/13/4798213/html/webhero/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Module/Taxamo/lib/Taxamo/Swagger.php on line 185

thank you,
Andy