Require Java 1.8 for disprover as well

The main ProB extension already requires Java 1.8, so this makes no
difference in the end.

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

@@ -11,7 +11,7 @@ Require-Bundle: org.eclipse.core.runtime;bundle-version="[3.5.0,4.0.0)",
  org.eclipse.ui;bundle-version="[3.5.0,4.0.0)",
  org.eventb.core.ast;bundle-version="[3.0.0,4.0.0)",
  org.rodinp.core;bundle-version="[1.7.0,2.0.0)"
-Bundle-RequiredExecutionEnvironment: J2SE-1.5
+Bundle-RequiredExecutionEnvironment: JavaSE-1.8
 Bundle-ActivationPolicy: lazy
 Bundle-Localization: plugin
 Export-Package: de.prob.eventb.disprover.core,

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

@@ -15,7 +15,7 @@ Require-Bundle: org.eclipse.core.runtime,
  org.rodinp.core;bundle-version="[1.7.0,2.0.0)",
  org.eventb.core;bundle-version="[3.0.0,4.0.0)",
  de.prob.ui;bundle-version="[7.4.2,7.5.0)"
-Bundle-RequiredExecutionEnvironment: J2SE-1.5
+Bundle-RequiredExecutionEnvironment: JavaSE-1.8
 Bundle-ActivationPolicy: lazy
 Bundle-Localization: plugin
 Bundle-Activator: de.prob.eventb.disprover.ui.DisproverActivator