get_apk.py is failing currently on checksums

[sylvestre]

I fixed the api version issue here: #49

Checksums change seems to be more than a one liner.```

raise HTTPError(http_error_msg, response=self)

requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://ftp.mozilla.org/pub/mobile/candidates/59.0.2-candidates/build1/android-api-16/multi/fennec-59.0.2.multi.android-arm.checksums

Checksums are available here: https://ftp.mozilla.org/pub/mobile/candidates/59.0.2-candidates/build1/

[sylvestre]

@JohanLorenzo is that something you could help with?
I updated my PR to disable the checksum verification:
#49