A Phase IIa Multi-centre Randomised Double-Blind Placebo-controlled Study to Assess the Efficacy, Safety and Pharmacokinetics of AZD8931 in Combination with Paclitaxel versus Paclitaxel alone in Patients with Metastatic, Gastric or Gastro-oesophageal Junction, Cancer who progress following First Line Therapy and are Ineligible for Treatment with trastuzumab by HER2 Status (SAGE)