Add a user-agent when using cURL

[LouWii]

Hi,

I recently ran into an issue with Minimee where it couldn't get files.
After some research, I found out that Minimee was blocked because cURL doesn't have any User-Agent parameter when trying to fetch files.

I think it's a good practice to set a User-Agent when using cURL.
It's common to block requests with no user agents as it's mostly crawlers or bots.
And you can also find it in your http logs if you ever need it at some point.

I'll be glad to make a pull request with that little modification.

What do you think ?