use pytest.ini to enable twisted plugin

pytest.ini

@@ -3,3 +3,5 @@
 usefixtures = chdir setlog
 python_files=test_*.py __init__.py
 addopts = --doctest-modules --assert=plain
+twisted = 1
+

tox.ini

@@ -16,7 +16,7 @@ deps =
     leveldb
     -rtests/requirements.txt
 commands =
-    py.test --twisted {posargs:scrapy tests}
+    py.test {posargs:scrapy tests}
 
 [testenv:precise]
 basepython = python2.7
@@ -35,7 +35,7 @@ basepython = python2.7
 commands =
     pip install -U https://github.com/scrapy/w3lib/archive/master.zip#egg=w3lib
     pip install -U https://github.com/scrapy/queuelib/archive/master.zip#egg=queuelib
-    py.test --twisted {posargs:scrapy tests}
+    py.test {posargs:scrapy tests}
 
 [testenv:py33]
 basepython = python3.3