make build-number a simple, increasing number as for android

deltachat-ios/Info.plist

@@ -28,7 +28,7 @@
 		</dict>
 	</array>
 	<key>CFBundleVersion</key>
-	<string>6.0</string>
+	<string>7</string>
 	<key>LSRequiresIPhoneOS</key>
 	<true/>
 	<key>LSSupportsOpeningDocumentsInPlace</key>