Updates related to test on Travis

There's two minor things in this PR:

1. I added the ``sandbox`` target to Travis as separate runs in the build matrix so that we know immediately when something is wrong with the sandbox build. I think that might help prevent people having issues because of requirement or installation issues. It's only a suggestion though. What are your thoughts @codeinthehole @maikhoepfel?

2. There was an issue with the ``spec`` plugin. Running the test on a fresh install would not expose ``--with-spec`` or ``--with-specplugin`` when running ``nosetests`` even through ``spec`` 0.9.7 was installed. The reason seemed to be that it doesn't work with ``nose>=1.2`` which is required for ``coverage`` to work. Running the ``spec`` commandline tool fails with a traceback (below). I updated ``requirements.txt`` to install directly from github because there is no newer version available. The latest version fixes the issue and exposes ``--with-specplugin`` as expected.
```
Traceback (most recent call last):
  File "/tmp/testenv/bin/spec", line 5, in <module>
    from pkg_resources import load_entry_point
  File "/tmp/testenv/local/lib/python2.7/site-packages/setuptools-0.6c11-py2.7.egg/pkg_resources.py", line 2607, in <module>
  File "/tmp/testenv/local/lib/python2.7/site-packages/setuptools-0.6c11-py2.7.egg/pkg_resources.py", line 565, in resolve
pkg_resources.DistributionNotFound: nose>=1.1,<1.2
```

Let me know what you think and if there's anything you want me to change.