Datetime is failing silently for one date
#### Code Sample, a copy-pastable example if possible

pd.to_datetime("06/2015") **# When my system date is 5/31/2016, it returns a string (06/2015)**
#### Expected Output

Timestamp('2015-06-01 00:00:00')
#### output of `pd.show_versions()`

'0.16.2'
