Travis: run buildout in newest mode.

That fixed the build for me locally.
Error on Travis:

$ bin/buildout
Getting distribution for 'zc.buildout<2.4.dev'.
While:
  Installing.
  Loading extensions.
  Getting distribution for 'zc.buildout<2.4.dev'.
An internal error occurred due to a bug in either zc.buildout or in a
recipe being used:
Traceback (most recent call last):
  File "/home/travis/build/zopefoundation/Zope/eggs/zc.buildout-2.9.3-py2.7.egg/zc/buildout/buildout.py", line 2123, in main
    getattr(buildout, command)(args)
  File "/home/travis/build/zopefoundation/Zope/eggs/zc.buildout-2.9.3-py2.7.egg/zc/buildout/buildout.py", line 637, in install
    self._load_extensions()
  File "/home/travis/build/zopefoundation/Zope/eggs/zc.buildout-2.9.3-py2.7.egg/zc/buildout/buildout.py", line 1163, in _load_extensions
    newest=self.newest, allow_hosts=self._allow_hosts)
  File "/home/travis/build/zopefoundation/Zope/eggs/zc.buildout-2.9.3-py2.7.egg/zc/buildout/easy_install.py", line 913, in install
    return installer.install(specs, working_set)
  File "/home/travis/build/zopefoundation/Zope/eggs/zc.buildout-2.9.3-py2.7.egg/zc/buildout/easy_install.py", line 714, in install
    for dist in self._get_dist(req, ws):
  File "/home/travis/build/zopefoundation/Zope/eggs/zc.buildout-2.9.3-py2.7.egg/zc/buildout/easy_install.py", line 570, in _get_dist
    dist = self._env.best_match(requirement, ws)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1040, in best_match
    dist = working_set.find(req)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/pkg_resources/__init__.py", line 672, in find
    raise VersionConflict(dist, req)
VersionConflict: (zc.buildout 2.9.3 (/home/travis/build/zopefoundation/Zope/eggs/zc.buildout-2.9.3-py2.7.egg), Requirement.parse('zc.buildout<2.4.dev'))

.travis.yml

@@ -10,7 +10,7 @@ notifications:
 
 install:
     - python bootstrap.py
-    - bin/buildout
+    - bin/buildout -n
 
 script:
     - bin/alltests