Add one to each BED mask interval to work with corrected augur mask

augur mask now reads in BED files following the standard expectation of a
zero-indexed, half-open interval such that the last value in each interval is
not included in the coordinates [1]. This commit updates the mask BED file for
this build to decrement each interval's start by one to compensate this change
in augur mask.

[1] nextstrain/augur#512 (comment)