Bug-fix release 3.1

This bug-fix release is about fixing a small screw-up in version 3.0, where we
introduced incompatible bmap format changes, but did not properly increase the
bmap format version number. Instead of making it to be version 2.0, we made it
to be version 1.4. The result is that bmap-tools v2.x crash with those
1.4-formatted bmap files.

This release changes the bmap format version from 1.4 to 2.0 in order to
lessen the versioning screw-up. Increased major bmap format version number will
make sure that older bmap-tools fail with a readable error message, instead of
crashing.

Thus, the situation as follows:

• bmap-tools v2.x: handle bmap format versions 1.0-1.3, crash with 1.4, and
  nicely exit with 2.0
• bmap-tools v3.0: handles all 1.x bmap format versions, exits nicely with 2.0
• bmap-tools v3.1: handles all bmap format versions