When passed a year, `parse_date_time()` returns the last day of the previous year.

[toph-allen]

When using lubridate 1.5.6, parse_date_time() returns an incorrect value when evaluating a year by itself. For example:

> parse_date_time("2016", orders = "y")
[1] "2015-12-31 UTC"
> parse_date_time("2016", orders = "y") %>% year()
[1] 2015

Reverting to 1.5.0 fixes this.

> parse_date_time("2016", orders = "y")
[1] "2016-01-01 UTC"
> parse_date_time("2016", orders = "y") %>% year()
[1] 2016

This doesn't seem to happen with other orders, e.g. "ymd".

[vspinu]

I think this was the consequence of #418.