Log Message:
Update ruby-patron to 0.9.1.

### 0.9.1

o Added ssl_version options `TLSv1_1`, `TLSv1_2`, `TLSv1_3` for explicitly
  forcing the SSL version

    * requires the appropriate versions of libCURL and OpenSSL installed to
      support these new options
    * reference: https://curl.haxx.se/libcurl/c/CURLOPT_SSLVERSION.html

o Added a new `:http_version` option with `HTTPv1_1` and `HTTPv2_0` values to
  explicitly set the HTTP version of HTTP/1.1 or HTTP/2.0

    * requires the appropriate versions of libCURL and OpenSSL installed to
      support these new options
    * reference: https://curl.haxx.se/libcurl/c/CURLOPT_HTTP_VERSION.html

o Updates the gem release procedure for more convenience, using the updated
  Rubygems.org tasks

o Update a few minor dependencies and documentation to be Ruby
  2.4.1-compatible, add 2.4.1. to Travis CI matrix

o Add `Session#download_byte_limit` for limiting the permitted download size.

  This can be very useful in dealing with untrusted download sources, which
  might attempt to send very large responses that would overwhelm the
  receiving client.

o Add `Patron.libcurl_version_exact` which returns a triplet of major, minor
  and patch libCURL version numbers. This can be used for more fine-grained
  matching when using some more esoteric Curl features which might not
  necessarily be available on libCURL Patron has been linked against.