Fix API documentation and build

[auroracramer]

Fix image embedding size in load_image_embedding_model() docstring, mock missing tensorflow.keras modules in doc/conf.py to fix API documentation build, and remove pin on sphinx version. Addresses #60 and #71.

[hohsiangwu]

Looks like the test failures are related to import skimage vs import skimage.transform as in here

Not sure if it is because skimage version is upgraded since we remove the limits on the version. Though why it's not failing in the previous build?

[hohsiangwu]

Except for the failing tests, other changes look good to me!

[auroracramer]

Thanks for taking a look into the build issues and finding this! I made an issue for it here: #73 and I'll merge in a fix to this branch.

[auroracramer]

As far why it didn't fail last time, it looks like it was because the last build to run was on May 29th, which was right before the travis-ci.org shutdown on May 31st. This repository was only recently migrated to travis-ci.com, so it looks that's why the change in skimage only appeared in this PR and when I renamed the main branch. What amazing timing 😂