Change OncoTree get script to be based on version instead of date of access

[brendanreardon]

Currently the get_oncotree.py script retrieves the version oncotree_latest_stable and saves the file by annotating the date of access. Kaelyn Nannini from Veteran Affairs recommended changing this to be based on the version from OncoTree.

OncoTree versions are retrievable with the api/versions endpoint and versions can be passed to /api/tumorTypes with the version argument.