Change to ~= requirements

[neubig]

Per the discussion here, we should be more rigorous in specifying the version numbers of the libraries that we depend on:
#403 (comment)

[tetsuok]

I've created a pull request #560 to use ~= operator, but it turned out this is not so straightforward. Transitive dependencies introduced by DataLab cause the CI jobs to fail when they run pip install .. We could revisit this issue once either the transitive dependencies or dependency on DataLab itself can be resolved.