Fixes 520 travis build ffmpeg failure

Current Travis YML was grabbing ffmpeg master branch to build, resulting in build failures based on ffmpeg changes. This pull changes the clone to get the tagged release number from ffmpeg instead. This does mean the project will need to manage the version of ffmpeg that is tested against, but that is probably not a bad thing. Will mean end users can be told ZoneMinder version x works with ffmpeg version y.