Feature/ar version check

Adds version number check and error message to AR module
Introduces python 3 compatibility for AR downsample
90% of AR tests to run in travis-ci
The discussion back in June/July timeframe concluded the AR downsample is
an optional feature, and therefore AR should not be in the conda recipe.

On Thu, Oct 9, 2014 at 6:29 AM, Damián Avila <notifications@github.com>
wrote:

> In scripts/travis_install:
>
> > @@ -32,6 +32,7 @@ EOF
> >  )
> >
> >  conda install --yes $DEPS_TEST
> > +conda install --yes abstract-rendering
>
> why here and not in the conda recipe?
>
> —
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> <https://github.com/ContinuumIO/bokeh/pull/1242/files#r18637356>.
>