Fix bundler error

MSP-9821

Fix

Bundler could not find compatible versions for gem "activerecord":
  In Gemfile:
    yard-metasploit-erd (~> 0.0.2) java depends on
      activerecord (< 4.0.0, >= 3.2) java

    metasploit_data_models (>= 0) java depends on
      activerecord (4.1.0)

by restricting activerecord's version in the gemspec to '< 4.0.0'.