Remove packetfu and pcaprub libaries

These should be handled by bundler's Gemfile.
Add Gemfile entry for PacketFu
Inch toward making modules more testable
Rspec: Ensure PacketFu is actually still available

PacketFu should be required from the gem, not from the shipped msf
library. Several modules depend on it being available, so this rspec
test mostly just ensures that Msf::Exploit::Capture mixin is still
around.
Land #1936, prefer PacketFu and pcaprub gems