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This reverts commit 01c88c1b.

Refs: #8263

Since then, we've bumped the memory requirements (both in our
documentation and on the test suite's "hardware") to 2 GB, so the
original reason for this change has gone away => let's go back to
a value that was tested and confirmed to work (as in: allow upgrading
a running Tails) on Wheezy, instead of a value that was tested on
Jessie, but only for checking for available upgrades.

The follow-ups will be:

 * the cool ideas posted on
   https://labs.riseup.net/code/issues/8263#note-1 will become a new,
   dedicated ticket (that has no reason to block the Tails 2.0 release);

 * #8083 and #7986 will tell us if the value this commit reverts to
   is OK.