.travis.yml - fix pspec->Attest->progressbar dependency installation

[asimihsan]

(WORK IN PROGRESS, first check that the Travis build passes)

In order to install progressbar we need to explicitly permit pip to
install external and unverified packages. Without this commit the Travis
build fails. This is likely linked to Travis updating their version of
pip to a version that now requires this behavior to be explicitly
permitted.

[asimihsan]

No luck; this commit fixes the dependency issue but hits the following error whilst using attest:

Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.6.9/bin/pspec", line 9, in <module>
    load_entry_point('pspec==0.0.2', 'console_scripts', 'pspec')()
  File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/pkg_resources.py", line 353, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/pkg_resources.py", line 2321, in load_entry_point
    return ep.load()
  File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/pkg_resources.py", line 2048, in load
    entry = __import__(self.module_name, globals(),globals(), ['__name__'])
  File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/pspec/__init__.py", line 2, in <module>
    from .collectors import run
  File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/pspec/collectors.py", line 2, in <module>
    from attest.utils import import_dotted_name, deep_get_members
ImportError: No module named utils

This commit certainly fixes something, so may be worth merging.