add attic dependency for build as a separate factor

this way we don't depend on attic for regular build, but we can still see proper test coverage

tox.ini

@@ -2,13 +2,15 @@
 # fakeroot -u tox --recreate
 
 [tox]
-envlist = py32, py33, py34, py35
+envlist = py{32,33,34,35}{,-attic}
 
 [testenv]
 # Change dir to avoid import problem for cython code. The directory does
 # not really matter, should be just different from the toplevel dir.
 changedir = {toxworkdir}
-deps = -rrequirements.d/development.txt
+deps =
+     -rrequirements.d/development.txt
+     attic: attic
 commands = py.test --cov=borg --pyargs {posargs:borg.testsuite}
 # fakeroot -u needs some env vars:
 passenv = *