fix tox regression / incompatible change

tests are broken for tox 3.2.x .. 3.4.0 versions:

/home/travis/build/borgbackup/borg/.tox/py34/bin/python: Error while
  finding spec for 'borg.archiver' (<class 'ImportError'>: No module
  named 'borg')

we see this breakage only on 1.0-maint branch, 1.1-maint is fine.

1.1-maint uses the src/ code layout, 1.0-maint does not. related?

requirements.d/development.txt

@@ -2,7 +2,7 @@ setuptools
 setuptools_scm
 pip
 virtualenv<14.0
-tox
+tox<3.2.0
 pytest!=3.3.0
 pytest-xdist
 pytest-cov