Image build issue fixes

[lemontheme]

Two minor adjustments were required in order for the image to build successfully. I'm too lazy to open a PR, but I thought I'd leave them here for future wanderers.

1. The miniconda install script URL redirects, causing the curl command in the Dockerfile to download an empty file. To fix this, replace it with RUN curl -L https://repo.continuum.io/miniconda/Miniconda2-latest-Linux-x86_64.sh -o /tmp/conda.sh. (The -L flag ensures that redirects are followed.)
2. This project uses Python 2 (Boo!), which a subdependency of the dependencies listed in requirements.txt has dropped support for. The fix here is to explicitly constrain the version of this subdependency to the last version that supported 2.7. Concretely, add imageio<2.7 to the top of requirements.txt.

The image should build now.

[ibarria0]

Thanks.

[PavanKumarPothula]

Thank you, @lemontheme. You are the messiah <3