src/Tweeper.php: use a minimal User-Agent string to fix scraping twitter.com

Twitter.com has started serving the user timeline via json when the user
agent is a modern browser, this breaks scraping in Tweeper which expects
html content.

Remove any version info from the User-Agent header used by Tweeper to
make twitter.com think it is talking with a very old browser, tricking
it into serving html content.

NOTE: Tweeper cannot just use the default User-Agent from the CURL
library because this would break scraping Facebook.com; using a minimal
but still browser-like User-Agent seems to be a viable common
denominator for all sites currently supported by Tweeper.