cope with MozillaOnline builds sending unexpected system capabilities strings

[bhearsum]

I just noticed that some mozillaonline builds are sending update URLs such as:
https://aus5.mozilla.org/update/6/firefox/68.0.2/20190813150448/winnt_x86_64-msvc-x64/zh-cn/release-cck-mainwinfull-mozillaonline/windows_nt%206.1.1.0.7601%20%28x64%29/iset:sse4_2%2Cmem:16341/mozillaonline/2019.7/update.xml

Because the strings in the system capabilities section are lowercase (iset:sse4_2%2Cmem:16341), our parsing is not working correctly: https://github.com/mozilla/balrog/blob/dfd356a96820a88f05195ec5cd8d536fc5830e8d/auslib/web/public/client.py#L38

Which causes exceptions like: invalid literal for int() with base 10: 'mem:16341'

We should adjust our parsing logic to lowercase those strings before comparison, but it would be good to sync up whatever is creating those mozillaonline URLs with the rest of the client side code.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1578474)