Store timestamp as POSIXct, not POSIXlt

[caijun]

To fix the Error: Column timestamp is a date/time and must be stored as POSIXct, not POSIXlt, we should store the timstamp variable as POSIXct, not POSIXlt, which also make it compatible with dplyr, ggplot2 etc. packages that requires putting POSIXct in a data frame. see issue tidyverse/dplyr#179

This PR may also fix the issue #2

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[amurthy-perf]

Dataset here (https://www.kaggle.com/boltzmannbrain/nab) needs this code change.
Looksgood.