DOC: Update source activate command to conda activate

[leonarduschen]

Zipline installation docs currently uses source activate and source deactivate - which only work on Linux and MacOS. So I thought we might want to change that to conda activate and conda deactivate as they are supported on all OS (available on Conda 4.4 onwards https://www.anaconda.com/blog/conda-4-6-release)

[coveralls]

Coverage remained the same at 88.26% when pulling 050365c on leonarduschen:update-conda-activate-command into 5e6c9fc on quantopian:master.

[leonarduschen]

I'm not sure why the test fails. I thought it was the Miniconda version used in AppVeyor test, but it doesn't explain why the test fails only in this particular environment:

Environment: PYTHON_VERSION=3.6, PANDAS_VERSION=0.22.0, NUMPY_VERSION=1.14.1, SCIPY_VERSION=1.0.0, STATSMODELS_VERSION=0.9.0, PANDAS_DATAREADER_VERSION=0.4.0, DASK_VERSION=0.17.1

[richafrank]

Thanks @leonarduschen ! I restarted the failed AppVeyor build, and it succeeded. And I think the note directive looks nice.