Re-implement FFMPEG::Movie and use ffprobe

Use yajl-ruby for JSON parsing
Provide ffprobe JSON output for RSpec fixtures

Additionally, refactor the Open3 stub methods to provide the corrected
stdout vs stderr. ffprobe provides its output on stdout instead of
stderr which is where it had been retreived previously.
ffprobe does not encounter these complexities

Because ffprobe does not return a decimal frame per second and instead
returns a rational number there is no need for this test.

Also, the colorspace returned in its own field. Therefore, regular
expression gymnastics are no longer requred.
Refactor specs to handle change in units

ffprobe returns its frame rate as a rational number as opposed to a
floating point (e.g., 24600/1469 instead of 16.75). This allows the
number to maintain a higher precision but is somewhat of a breaking
change for the specs and thus the API.

ffprobe's default units are bit/s instead of the old ffmpeg output being
in kb/s. Again, higher precision but a breaking change for the specs and
thus the API.

ffprobe does not provide a property like FFMPEG::Movie#video_stream or
FFMPEG::Movie#audio_stream. Therefore, an attempt to emulate it and
provide backwards compatibility is provided.