A Phase I Randomized, Observer-Blind, Controlled, Dose Escalation Trial of the Safety and Tolerability of Two Intramuscular Doses of DPX-RSV(A), a Respiratory Syncytial Virus (RSV) Vaccine Containing RSV SH Antigen and a Novel Adjuvant DepoVax, or SH Antigen Co-administered With Aluminum Hydroxide, or Placebo to Healthy Adults ≥50–64 Years of Age