Add metasploit-model as a direct dependency

MSP-10905

Metasploit::Model constants need to be accessible without the db group
installed.

Gemfile.lock

@@ -5,6 +5,7 @@ PATH
       activesupport (>= 3.0.0, < 4.0.0)
       bcrypt
       json
+      metasploit-model (~> 0.25.6)
       meterpreter_bins (= 0.0.6)
       msgpack
       nokogiri

metasploit-framework.gemspec

@@ -55,6 +55,8 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency 'bcrypt'
   # Needed for some admin modules (scrutinizer_add_user.rb)
   spec.add_runtime_dependency 'json'
+  # Metasploit::Model constant used with and without database gems
+  spec.add_runtime_dependency 'metasploit-model', '~> 0.25.6'
   # Needed for Meterpreter on Windows, soon others.
   spec.add_runtime_dependency 'meterpreter_bins', '0.0.6'
   # Needed by msfgui and other rpc components