Add check for Bash >= 3.1 (required for += array assignment).

Add check for Bash >= 3.1 (required for += array assignment).

Browsing through the changelog
  http://git.savannah.gnu.org/cgit/bash.git/tree/CWRU/changelog
the change that added += was
  2004/11/05
  general.c
  - change `assignment()' to accept `+=' assignment operator
Looking through the released tarballs
  ftp://ftp.gnu.org/gnu/bash/
this change made it in between release 3.0.16 and 3.1.