tox.ini: skip default install

We use "--deps = --editable=.[test]" to achieve three goals:

* make tahoe and it's dependencies available for tests
* use --editable, which is faster and allows "coverage run" to get the
  source filenames right
* use the [test] extra, which includes "mock"

Tox's default install command does the first, but doesn't use
--editable, so when the "deps" stage comes around, there's already a
non-editable install in place. It seems to get the [test] extra right,
but it doesn't wind up with an editable install.

So we disable the default install command and rely on the "deps" clause
instead.

tox.ini

@@ -8,6 +8,8 @@ envlist = py27
 
 [testenv]
 passenv = USERPROFILE HOMEDRIVE HOMEPATH
+# the default install is non-(--editable), and would displace our -e .[test]
+skip_install = True
 deps = --editable=.[test]
 commands =
          tahoe --version
@@ -16,6 +18,7 @@ commands =
 [testenv:deprecations]
 basepython=python2.7
 passenv = USERPROFILE HOMEDRIVE HOMEPATH
+skip_install = True
 deps = --editable=.[test]
 setenv =
          PYTHONWARNINGS=default::DeprecationWarning
@@ -27,6 +30,7 @@ basepython=python2.7
 passenv = USERPROFILE HOMEDRIVE HOMEPATH
 setenv =
          PYTHONWARNINGS=default::DeprecationWarning
+skip_install = True
 deps =
      --editable=.[test]
      git+https://github.com/twisted/twisted
@@ -58,5 +62,7 @@ commands =
 
 [testenv:docs]
 deps = sphinx
+# normal install is not needed for docs, and slows things down
+skip_install = True
 commands =
          sphinx-build -b html -d {toxinidir}/docs/_build/doctrees {toxinidir}/docs {toxinidir}/docs/_build/html