Use ms constants only if we have curl >= 7.16.2

This should avoid any warnings, as the constants `CURLOPT_CONNECTTIMEOUT_MS` and `CURLOPT_TIMEOUT_MS` are only available on curl >= 7.16.2

If we are below that version (should we still support it ? it is getting kinda old...), use the constants in seconds rather than milliseconds (`CURL_CONNECTTIMEOUT` and `CURL_TIMEOUT`), with a multiplication per 1000 for the `handshakeTimeout` parameter.

And I also took the liberty to remove the `require` in top of the `Client` class. We're using composer, so autoloading is not really our problem.

poke @guillaumepotier