CI: ensure proper coverage of different pyarrow versions
Closes https://github.com/pandas-dev/pandas/issues/29223, closes https://github.com/pandas-dev/pandas/issues/26716
Currently, pyarrow is included in the following builds:

- azure-macos-36 as `pyarrow>=0.12.0` -> this currently leads to version 0.13.0 in practice
- azure-windows-36 as `pyarrow>=0.12.0` -> this currently leads to version 0.13.0 in practice
- travis-37 as `pyarrow` -> this currently leads to 0.13.0 in practice.

The reason for this is that pyarrow is only at 0.13 on the default Anaconda channel. So for a more recent version, we will need one of our builds to use conda-forge.

We should also test the oldest supported version (0.12.0), this I added to `azure-36-minimum_versions` OK, the build using conda-forge to have pyarrow 0.15 seems to work. 
I could also make this a new build instead of adapting an existing one (but to me that doesn't seem worth it)

The build with the minimum of 0.12 seems to be using arrow-cpp / pyarrow from conda-forge, because 0.12 is not available on the default channel (I think only 0.9 and 0.13). I am not sure this mixing of channels will work out, but it seems to go fine (although I can't check the tests actually run and are not skipped due to an import error, since the printing of skipped tests is not working anymore -> https://github.com/pandas-dev/pandas/issues/30040) cc @jreback @datapythonista @alimcmaster1  thanks @jorisvandenbossche 