changed precison threshold for LogSoftmax test

The previous threshold was insufficient and the test occasionally failed. I ran it a large number of times (~1e5) and the actual error reached ~ 2x1e-4, so I propose 3*1e-10 as the new safer threshold.
Update test.lua
Merge pull request #16 from GeorgOstrovski/patch-1

changed precison threshold for LogSoftmax test