Deprecation warning for -b video bitrate option

[florish]

I keep getting these warnings on setting the output video bitrate when using the:video_bitrate option in the FFMPEG::Movie#transcode method:

Please use -b:a or -b:v, -b is ambiguous

Not a critical bug, but something that might become one in the future. Apparently the ffmpeg developers changed the preferred syntax for the -b (video bitrate) option. I don't know why exactly, as there was already a -ab audio bitrate option, but this error might prevent the gem from working properly in the future.

[dbackeus]

Thanks for reporting this. The gem is always tested against a specific ffmpeg version (currently 0.10.2) so when the time comes to bump the version the parameter syntax will be updated as well.

[florish]

Ok, great. Just to be clear: I'm using ffmpeg 0.10.2, so it's something that's already deprecated in the version the gem is tested against.

[dbackeus]

The old style still worked but I updated to use the new syntax now.

Also added checking of bitrate results in transcoder spec to make sure something would catch it if they would change it in the future (I never saw the warnings since test output is silent).