keep tests simple by always adding attic depends

note that we do not depend on attic to build borg, just to do those
tests. if attic goes away, we could eventually do this another way or
just stop testing this altogether.

tox.ini

@@ -2,7 +2,7 @@
 # fakeroot -u tox --recreate
 
 [tox]
-envlist = py{32,33,34,35}{,-attic}
+envlist = py{32,33,34,35}
 
 [testenv]
 # Change dir to avoid import problem for cython code. The directory does
@@ -10,7 +10,7 @@ envlist = py{32,33,34,35}{,-attic}
 changedir = {toxworkdir}
 deps =
      -rrequirements.d/development.txt
-     attic: attic
+     attic
 commands = py.test --cov=borg --pyargs {posargs:borg.testsuite}
 # fakeroot -u needs some env vars:
 passenv = *