Convert timestamp to float before using with arrow

arrow>=0.15.0 doesn't accept timestamps given as `str`. Unfortunately
`json` used in ChangeSet doesn't preserve `float` type in dictionary
keys and always converts it to `str`. This leaded to feeding `arrow`
with unsupported strings.

Read note at: https://docs.python.org/3/library/json.html#json.dumps for
further explanation.

This fixes #271 and #274

topydo/commands/RevertCommand.py

@@ -98,7 +98,7 @@ class RevertCommand(Command):
         num = 1
         for timestamp, change in self._backup:
             label = change[2]
-            time = arrow.get(timestamp).format('YYYY-MM-DD HH:mm:ss')
+            time = arrow.get(float(timestamp)).format('YYYY-MM-DD HH:mm:ss')
 
             self.out('{0: >3}| {1} | {2}'.format(str(num), time, label))
             num += 1