Wapm update notification doesn't invalidate cache

[MarkMcCaskey]

The Wapm update notification will display the wrong thing if the value stored there is out of date.

For example if the value in the $WASMER_DIR/.wapm_update.json is 1.0.0-alpha3 and we're on 1.0.0-beta1, it will say to update to an older version because it's different and we don't invalidate the old value stored there.

We should put a relatively short lifetime on the value stored there to prevent this from happening.