Bump src/executor

Bump cloudfoundry-incubator/executor:
  Ted Young:
     Update deps to latest, pulling in gofileutils ( Travis Success: https://travis-ci.org/cloudfoundry-incubator/executor/builds/20959924 )
     Remove unused import ( Travis Unknown )
     Use gofileutils instead of shelling out to `cp` ( Travis Failed: https://travis-ci.org/cloudfoundry-incubator/executor/builds/20957514 )
     Extractor is not responsible for removing archive ( Travis Unknown )
     Extractor supports .tgz archives ( Travis Unknown )

Signed-off-by: Aram Price <aprice@pivotallabs.com>