Randomized, Active-Comparison, Double-Blind, Phase 2 Study to Assess the Safety and Immunogenicity of Anthrax Vaccine Adsorbed (BioThrax®) Without and With CPG 7909 Adjuvant (AV7909 Anthrax Vaccine), Using a Post-Exposure Prophylaxis Dosing Regimen in Adults 66 Years of Age or Older in Stable Health in Comparison to Adults 18-50 Years of Age in Stable Health