If your server supports PHP, leaving it at version 5.5 shouldn't make any difference to processing the php file included with the Hike template, providing your hosting server has the function of PHP(send) configured within their server php.ini file, just changing what you need to change in the php file using the documentation that came with the template, it will work.
PHP has nothing to do with Webplus, Webplus creates the site in an HTML format, PHP is more a server scripting language that executes what ever commands are sent to the server via written scripts, in this case, the contact.php file within the template download.
Also, if your hosting provider says you must upgrade from 5.5 to 5.6 or 7.0, then doing so should not change anything at your end and if they are a hoster of any kind of reputation, there should be no charge to yourself to upgrade PHP on THEIR SERVERS, it is done more of a security policy and should not reflect any charges to you as a paying hosting customer. If they tell you otherwise, my suggestion is to change providers.
Hope this helps