specs2-tests SPECS2-3.9.4

Maintenance version (3.9.3 was incorrect)

Improvements

• core when checking for the equality of 2 sets of lines only run the edit distance difference on them if they are different.
  this improves build times

Fixes

• core set the minimum number of execution threads to 4 to avoid possible deadlocks on single core machines (cf some CI environments)
• core use the "user" execution environment to execute examples instead of specs2 own execution environment