The `psutil.BOOT_TIME` attribute appears to be deprecated, and returns an `AttributeError` when called.
Even though the psutil documentation states that old attributes should be callable, one should use `psutil.boot_time()` function instead.
In some version of psutil (e.g. 2.1.1), both appear callable. In old versions (e.g. 1.2.1), `psutil.BOOT_TIME` is but `psutil.boot_time()` is not. In the newest version (4.3.0), only the latter can be called.
This bug would introduce an Internal Server Error (Error 500) when accessing the Monitor tool in the YunoHost admin panel.
Since monitoring statistics was not finished - and by the way not usable,
it inverts the behavior of monitor_enable to disable it by default.
See #395 for more details.
