Mention two PHP extension dependencies in the README.

[orlitzky]

The PHP cURL extension is needed for the library itself, as can be
seen e.g. by the use of all the CURL_FOO constants. When compiling PHP
from source, this is an optional extension, so we mention it in the
README.

The test suite requires the additional JSON extension, for the
json_decode() function called throughout the suite. This is now
mentioned in the README as well.

[coveralls]

Coverage remained the same when pulling 2324774 on orlitzky:master into d02c5d3 on rmccue:master.

[miquelfire]

This library doesn't need cURL, it's an optional dependency. If cURL is not installed, it uses PHP's sockets.

[orlitzky]

You're right, sorry! I'll see if I can nuke that commit with another one without the cURL stuff.

[rmccue]

Thanks! 🎄 🎁

[coveralls]

Coverage increased (+0.01%) when pulling a9b88f8 on orlitzky:master into d02c5d3 on rmccue:master.