Update pants bootstrap script to be agnostic to sed version.
Update pants bootstrap script to be agnostic to sed version.

Reviewed at https://reviews.apache.org/r/40204/

medallia/aurora,thinker0/aurora,medallia/aurora,mschenck/aurora,mschenck/aurora,protochron/aurora,mschenck/aurora,crashlytics/aurora,apache/aurora,medallia/aurora,apache/aurora,medallia/aurora,apache/aurora,crashlytics/aurora,mschenck/aurora,mschenck/aurora,thinker0/aurora,crashlytics/aurora,apache/aurora,medallia/aurora,thinker0/aurora,thinker0/aurora,rdelval/aurora,protochron/aurora,protochron/aurora,crashlytics/aurora,apache/aurora,crashlytics/aurora,rdelval/aurora,rdelval/aurora,rdelval/aurora,rdelval/aurora,thinker0/aurora,crashlytics/aurora,mschenck/aurora,protochron/aurora,protochron/aurora,apache/aurora,rdelval/aurora,medallia/aurora,protochron/aurora,thinker0/aurora