I found another way. I used the following option:

--preferred-challenges="dns"

The entire command looked like this:

certbot certonly --manual -d theos-blog.com -d infosec.theos-blog.com --preferred-challenges="dns"

When you run that command, you'll get output similar to the following:

Please deploy a DNS TXT record under the name
_acme-challenge.theos-blog.com with the following value:
 
<random looking string 1>
 
Once this is deployed,
-------------------------------------------------------------------------------
Press Enter to Continue
 
-------------------------------------------------------------------------------
Please deploy a DNS TXT record under the name
_acme-challenge.infosec.theos-blog.com with the following value:
 
<random looking string 2>
 
Once this is deployed,
-------------------------------------------------------------------------------
Press Enter to Continue

Navigate to your CloudFlare admin page and select DNS from the top menu.

In the dropdown under DNS Records, choose TXT. Enter the name provided under Name, and the random looking string under Value, then click on Add Record. It should look similar to the following:

Back on your server, hit Enter to allow the process to continue.

If all goes well, you'll get output like below:

Waiting for verification...
Cleaning up challenges
 
IMPORTANT NOTES:
 - Congratulations! Your certificate and chain have been saved at
   /etc/letsencrypt/live/infosec.theos-blog.com/fullchain.pem. Your cert will expire on 2017-11-08. To obtain a new or tweaked version of this certificate in the future, simply run certbot again. To non-interactively renew *all* of your certificates, run "certbot renew"
 - If you like Certbot, please consider supporting our work by:
   Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate
   Donating to EFF:                    https://eff.org/donate-le

Now, restart your server, nginx in my case:

systemctlhttpd restart

Refresh your page in your browser, and you're up and running!