Updating v0.5.0 notes in README

Also fixing minor bug in example usage code
Using 'ensure' to make sure Reader file is closed if exception occurs in given block

Not sure exactly how to test this at the moment
Adding :float as a synonym for :float_32 when creating a Format

Since 32 and 64-bit floats are the same in Ruby, this lets you
avoid a meaningless distinction in certain cases, such as reading
from a file as floating point.
Updating gemspec to get the version from WaveFile::VERSION

Removes duplication
Updating the list of compatible Ruby versions in the README
Changing Writer.sample_frames_written to total_sample_frames

Also changing duration_written to total_duration. Not totally
thrilled about these names, but they seem a bit more future proof,
and are consistent with Reader.total_sample_frames.