Add '--ignore-installed six' to pip install arguments.

[cmars]

This was necessary to avoid conflicts with the python3-six Ubuntu
package, which is a dependency of python3-pip.

[cmars]

Was getting this install hook error in all my layered charms prior to this fix: https://paste.ubuntu.com/14582988/

[1stvamp]

I'm getting a similar error for yaml in #27

[marcoceppi]

This is because of a recent release of pip. This patch address some of the
side effects.

On Thu, Jan 21, 2016, 3:14 AM Wes Mason notifications@github.com wrote:

I'm getting a similar error for yaml in #27
juju-solutions#27

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juju-solutions#26 (comment)
.

[marcoceppi]

I've attempted to address this here juju/charm-tools#90 though this isn't the only way to resolve this. @johnsca would this be better to have the base layer define the pip version or should we continue to include this in charm-tools as juju/charm-tools#90 has done?

[johnsca]

@marcoceppi Yes, I think it would be better for the pip requirement to explicitly be in the base layer, and version-locked like the other wheelhouse deps.

[marcoceppi]

I'll update the pull requests, thanks