Revert "run tests without depending on an already installed instance"

Breaks CLI (and probably other modules that use `dist.version`):

    [*]~/d/isso:master> isso -c sample.cfg run
    Traceback (most recent call last):
    File "/home/.../dev/isso/bin/isso", line 9, in <module>
        load_entry_point('isso==0.8.dev0', 'console_scripts', 'isso')()
    File "/home/.../dev/isso/isso/__init__.py", line 198, in main
        parser.add_argument('--version', action='version', version='%(prog)s ' + dist.version)
    AttributeError: 'module' object has no attribute 'version

This reverts commit 4e5e8c44f77ae6a3aacbc2cfd78c4965c60b33d5.