downloading webm instead of MP4 from gfycat

* Ripme version:
* Java version: 1.7.69
* Operating system: w10


## Expected Behavior

I expect preferring mp4 files over gif would also prefer mp4 files over webm

## Actual Behavior

webm files are downloaded from gfycat even though replacing the .webm in the URL with .mp4 results in a valid mp4 file.
