test out 2.18-SNAPSHOT for surefire

hopefully, this resolves travis hangings.
add surefire snapshot repo
Reduce fork count and reuse when running under travis ci.
Update SingularityUIForDevelopers.md
Makes sandbox work for default executor
Merge branch 'master' into hs_staging
Merge pull request #278 from hgschmie/surefire-fix

Reduce fork count and reuse when running under travis ci.
Merge branch 'master' into hs_staging