Is needed only for python 3.2 support.
For normal development, we expect you have py34+ for borg 1.1.
For vagrant, it is still needed because of older VMs like wheezy (py32).
Not needed for Travis-CI any more, we moved to trusty VMs (py34) there.
use python 3.5.2 on trusty travis-ci machines
their 3.5(.0) misses lzma support at the moment.
after this is fixed, we likely want to go back to 3.5(.0) to run the tests.
it seems to keep .tox envs even if one removes stuff from requirements file.
if that stuff causes troubles, as here with pytest-benchmark, the troubles stay...
pytest-benchmark (and its dependency "statistics") needs py 3.3+.
we do not want to run benchmarks on travis anyway, but even its presence
makes trouble due to this, so we just remove it from there again.
as --benchmark-skip is also removed from tox configuration, this means benchmarks
are run by default when pytest-benchmark is installed.
