Add ZDI authority

MSP-9325
Filter invalid children from union operations.

MSP-9277

Filter invalid children before adding them to
Metasploit::Model::Search::Operation:Union#children so that invalid
children won't stop the union from validating, which can cause
deprecated operators to fail.

(cherry picked from commit abb187e)
Fix format of ZDI designation sequence

MSP-9325

Number after 2 digit year should only have 3 digits, not 4.
Add ruby 2.0+ to travis-ci

Add 2.0 and 2.1 to .travis.yml to check compatibility with ruby 2.0 and
ruby 2.1 since it's not clear if we'll next jump to 2.0 or 2.1 if we
stick with MRI.
Merge branch 'feature/ruby-2.0+' into feature/module-cache-construction
Use coveralls

MSP-9399
Add coveralls badge to README

MSP-9399
Merge branch 'feature/coveralls' into feature/module-cache-construction
Fix branches in badges

MSP-9399
Merge pull request #12 from rapid7/feature/zdi-authority

ZDI Authority
MSP-9325
Merge branch 'feature/module-cache-construction' into feature/firefox-and-nodejs

Conflicts:
	lib/metasploit/model/version.rb