Fix latest version checks

[cyberMilosz]

Our latest version check currently relies on a file that never existed, does not exist, and will never exist:

drozer/src/drozer/meta.py

Lines 52 to 61 in 9737ce2

	def latest_version():
	try:
	xml = urlopen(Request("https://www.withsecure.com/products/drozer/community-edition/manifest.xml", None, {"user-agent": "drozer: %s" % version}), None, 1).read()
	doc = ElementTree.fromstring(xml)
	return max(map(lambda n: Version(n.text[1:], n.attrib['release_date']), doc.findall('version')))
	except HTTPError:
	return None
	except URLError:
	return None

Let's use something that exists instead - like the GitHub API! Should be as simple as parsing https://api.github.com/repos/WithSecureLabs/drozer/releases/latest