Replace mock with double in specs

mock is deprecated - https://www.relishapp.com/rspec/rspec-mocks/docs
Remove rpsec should_not raise_error deprecations

Checking that a specifc error is not raised is deprecated in rspec:
https://github.com/rspec/rspec-expectations/pull/244
Update test gems

This should not affect core Metasploit Framework as it only updates gems
in the test group (and dependencies of those gems).
Update active* and other gems
Older versions of rake break specs
Grammar fixes in descriptions
Land #2341, @todb-r7's grammar fixes
Land #2335, Gemfile updates (run bundle install!)

FAQ:

Q: I get an error telling me to run bundle install when I try to start
msfconsole. What do I do?

A: Type: "bundle install" (or msfupdate which runs it, too)
Fix requires in new post modules
Land #2347, nail down the require paths
Fix additional issues in the python meterpreter.
Land #2349 pymeterp fixes