Update docs' lists of supported HTTP clients

I noticed these were out of sync, especially weirdly one mentioning boto and the other mentioning boto3. I figure the list in the README is redundant when the Installation docs section lists the supported libraries, so I've removed it. I also:

* alphabetically sorted the list
* Highlighted the library names as code
* Added both `boto` and `boto3` to the list since there is support for both
* Removed the comment about Tornado's AsyncHTTPClient since that's an implementation detail plus the patch happens on a couple different classes
* Removed the note about `http.client` being part of Python 3, because everyone is Python 3 these days anyway :)

README.rst

@@ -41,19 +41,6 @@ VCR.py will detect the absence of a cassette file and once again record
 all HTTP interactions, which will update them to correspond to the new
 API.
 
-Support
--------
-
-VCR.py works great with the following HTTP clients:
-
--  requests
--  aiohttp
--  urllib3
--  tornado
--  urllib2
--  boto3
-
-
 License
 =======