Fix specifications

After moving all requirements to lib/sword.rb they were missing in
specifications, so I've added a lot of 'require' statements in specs.
Probably should move it all back where it was.
Fix Sword::Bundle#invoke argument handling

Now it accepts any number of arguments and redirects them to super
method. Before it didn't accept arguments and thus only supported tasks
invocation without arguments.
Add Ruby 2.0 back into .travis.yml