PrimeGrid 3*2^n-1 switched to +1

[Free-DC]

We've switched 3*2^n-1 subproject, which has been completed to n=5M and had no more work, to 3*2^n+1. The tasks are significantly shorter now (in the range of minutes), but it will change quite rapidly as we progress through the test space.

More... (http://www.primegrid.com/forum_thread.php?id=1989)