CI: faster CI builds

[jreback]

#7971 recently changed use to use conda on the 5 main builds

installs now take 300+ seconds, vs 100+ before.

I think a big cause is the installation of some basic libraries that conda installs by default
e.g. Python 2.7.8. But I don't think this is necessary and maybe can skip some of these

[jreback]

@cpcloud

[cpcloud]

We can use miniconda3 for the Python 34 builds and miniconda for Python 27 but for the others there's no easy way to avoid installing Python

[cpcloud]

They use hard links so there should only be the initial installation and that's it, but no matter what I think we'll have to use the Python included wjth conda

[jreback]

ok whatever y can do would be gr8

[cpcloud]

200 seconds also seems like a huge increase just to install a prebuilt Python.

[jreback]

it may not be that (itself) but everything
look at builds right before the conda update was merged

[cpcloud]

is this still an issue?