Mention that Down::Http now uses 10x less memory
Mention that Down::Http now uses 10x less memory

HTTP.rb 3.2.0 now allocates 10x less memory than before when retrieving
response body, which means the Down::Http backend also uses 10x less
memory. Recommend upgrading to HTTP.rb 3.2.0 and mention the difference
in memory usage between Down::Http and Down::NetHttp backends.

Also recommend using the latest Down version in the README, due to
various improvements, especially towards memory usage of the Down::Http
backend (unrelated to the HTTP.rb upgrade).

janko-m/down