Bump all the version numbers for next version 3.1.2

de.bmotionstudio.gef.editor/META-INF/MANIFEST.MF

de.bmotionstudio.rodin/META-INF/MANIFEST.MF

de.prob.core.tests/META-INF/MANIFEST.MF

de.prob.core/META-INF/MANIFEST.MF

de.prob.eventb.disprover.core/META-INF/MANIFEST.MF

de.prob.eventb.disprover.ui/META-INF/MANIFEST.MF

de.prob.plugin/META-INF/MANIFEST.MF

de.prob.symbolic/META-INF/MANIFEST.MF

de.prob.ui/META-INF/MANIFEST.MF

de.prob2.disprover.feature/feature.xml

de.prob2.feature/feature.xml

de.prob2.symbolic.feature/feature.xml