AttributeError: 'module' object has no attribute 'is_tool_load_error'

Hi!

Firstly, thank you for this tool! It can help us a lot improving our tool development's workflow.

Here is the description of the issue:

I am trying to install Planemo in a virtual environment. Usual stuff. It installs properly, but when I want to launch Planemo (any command), I get this output:

```
(env_galaxy_clean)[fnc-galaxy@bioinf-gal001 galaxy]$ planemo t
Traceback (most recent call last):
File "/data/galaxy/env_galaxy_clean/bin/planemo", line 11, in
sys.exit(planemo())
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/click/core.py", line 700, in __call__
return self.main(*args, **kwargs)
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/click/core.py", line 680, in main
rv = self.invoke(ctx)
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/click/core.py", line 1022, in invoke
cmd_name, cmd, args = self.resolve_command(ctx, args)
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/click/core.py", line 1061, in resolve_command
cmd = self.get_command(ctx, cmd_name)
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/planemo/cli.py", line 101, in get_command
return name_to_command(name)
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/planemo/cli.py", line 86, in name_to_command
mod = __import__(mod_name, None, None, ['cli'])
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/planemo/commands/cmd_test.py", line 7, in
from planemo import galaxy_config
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/planemo/galaxy_config.py", line 23, in
from planemo.shed import tool_shed_url
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/planemo/shed/__init__.py", line 28, in
from planemo.tools import load_tool_elements_from_path
File "/data/galaxy/env_galaxy_clean/lib/python2.7/site-packages/planemo/tools.py", line 7, in
is_tool_load_error = loader_directory.is_tool_load_error
AttributeError: 'module' object has no attribute 'is_tool_load_error'
```

> Your operating system name and version, versions of other relevant software such as Galaxy or Docker.

CentOS release 6.7 (Final)

> Any details about your local setup that might be helpful in troubleshooting.

We have a virtualenv at the same level than the Galaxy folder. We are using it for Planemo and Galaxy.

> Detailed steps to reproduce the bug.

This is the tricky part. I have absolutely no idea how to reproduce it. I have recreated a virtualenv, and planemo is installing and running properly :s.

I tried to uninstall all the Planemo package dependencies, and let pip/Planemo install them, and I still have the same error :s.

Thanks for the help/resolve on this!