skip the benchmarks by default, especially on travis

tox.ini

@@ -11,6 +11,6 @@ changedir = {toxworkdir}
 deps =
      -rrequirements.d/development.txt
      attic
-commands = py.test --cov=borg --pyargs {posargs:borg.testsuite}
+commands = py.test --cov=borg --benchmark-skip --pyargs {posargs:borg.testsuite}
 # fakeroot -u needs some env vars:
 passenv = *