It sounds like the plugin you installed is likely the cause of the issues you’re running into. I would recommend disabling the plugin if you can (if not, you can delete the plugin’s directory completely). A clue that this is not directly related to cURL is that you mentioned jQuery errors. In general javascript errors would indicate an error in the front-end code being run by the browser and not the backend code run on your server. PHP’s curl function would only be called on the server side so my best guess is that the plugin you installed had added code to your site that has broken other things.

For reference and for anyone else who encounters a similar error, curl for PHP can be installed

On Ubuntu 16.04 with:

apt-get update
apt-get install php7.0-curl

On Ubunu 14.04 with:

apt-get update
apt-get install php5-curl