tox: add 'skipsdist=True', hoping this will fix buildbot

There appears to be a bug in setuptools, triggered by running "python
setup.py sdist" with setuptools==11.3 in that python's environment, on a
project whose setup.py has a setup_requires= that requests setuptools >=
28.8.0. When setuptools is upgraded from inside setup.py, it gets into a
weird hybrid state where it's using setup() keyword-argument plugins from the
newer setuptools, but those plugins reference functions that aren't present
in the older setuptools, and the sdist command fails with an import
error (module object has no attribute 'check_specifier').

We don't actually need the sdist: all our tox test environments use
"skip_install = true", because we install tahoe via the "deps" line (so we
can get the `[test]` extra, and get a faster symlink-ish "editable" install).
That install uses "pip", which uses the pip inside the new virtualenv, which
either uses a newer version of setuptools (dependent upon what version of
"virtualenv" was installed in the parent environment, next to tox) or somehow
allows setuptools to be upgraded without exposing this weird broken hybrid
state.

Either way, skipping the sdist seems to fix this problem.

refs ticket:2910
https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2910

tox.ini

@@ -9,6 +9,7 @@ twisted = 1
 [tox]
 envlist = py27
 minversion = 1.7
+skipsdist = True
 
 [testenv]
 passenv = TAHOE_LAFS_TRIAL_ARGS USERPROFILE HOMEDRIVE HOMEPATH