tox.ini: use --editable to work around old-pip bug

One of the buildslaves (Ubuntu wily 15.10) has a very old pip-1.5.6,
which doesn't know how to "pip install" a filepath+extra (like
".[test]") unless --editable is also used.

It's convenient to have --editable set anyways (so you can do subsequent
narrow testing without re-running tox, by running ".tox/py27/bin/trial
TESTCASE" or use .tox/py27/bin/activate), so changing the dependency
from ".[test]" to "--editable=.[test]" is the easiest way to work around
that older buildslave. (I could also have upgraded the buildslave to use
a newer pip, but 15.10 is pretty recent and other people will probably
hit this too, so this way it's fixed for everybody).

refs ticket:2776

tox.ini

@@ -8,7 +8,7 @@ envlist = py27
 
 [testenv]
 passenv = USERPROFILE HOMEDRIVE HOMEPATH
-deps = .[test]
+deps = --editable=.[test]
 commands =
          tahoe --version
          trial --rterrors {posargs:allmydata}
@@ -16,7 +16,7 @@ commands =
 [testenv:deprecations]
 basepython=python2.7
 passenv = USERPROFILE HOMEDRIVE HOMEPATH
-deps = .[test]
+deps = --editable=.[test]
 setenv =
          PYTHONWARNINGS=default::DeprecationWarning
 commands =
@@ -28,7 +28,7 @@ passenv = USERPROFILE HOMEDRIVE HOMEPATH
 setenv =
          PYTHONWARNINGS=default::DeprecationWarning
 deps =
-     .[test]
+     --editable=.[test]
      git+https://github.com/twisted/twisted
      git+https://github.com/warner/foolscap
 commands =