Include a "simple" User-Agent string as well as the JSON

To facilitate detection of pip in scenarios where parsing JSON isn't
easy but simple string matching is, change the User-Agent to be of
the form "pip/{version} {json}" instead of just "{json}".

The contents of the JSON data has not changed.
Merge pull request #2086 from dstufft/simpler-json-prefix

Include a "simple" User-Agent string as well as the JSON