Union on a sequence of Intervals uses reduceRight, exhausting the stack

[kwalcock]

The code is here:

common/src/main/scala/ai/lum/common/Interval.scala

Line 399 in b7c0b70

sorted.reduceRight(_ union _)

It is causing reach to crash on large nxml input files with too many sections.

See also allenai/openie-standalone#36

Changing to reduceLeft seems to fix the problem. A PR is being prepared.