DOC: update install.rst to include ActivePython distribution
PR associated with [Issue #19845](https://github.com/pandas-dev/pandas/issues/19845#issuecomment-367879282): update Installation webpage to include ActivePython distribution 

per @jreback 

- [ ] closes #19845
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
