Relax the ureq dependency version requirement to 0.8.1 or greater.

[lawlawlaw]

No description provided.

[lawlawlaw]

This is required to use bitcoind with rocket version 4 (https://github.com/SergioBenitez/Rocket/releases/tag/v0.4.11) due to a dependency conflict. Rocket version 4 uses ring version 0.13.x.

[RCasatta]

Hi, thanks for your PR.

I see some issues:

1. if you rework your PR adding commits but changing the same line you should squash your changes (make just 1 commit)
2. ring version with ureq 0.8.2 looks 0.14 not 0.13.x as you stated
3. I am planning to do a last release with MSRV 1.41.1 and then switch to an higher one with ureq 2.x (this is one of the reason Fix stdweb audit advisotry RUSTSEC-2020-0056 bitcoindevkit/bdk-cli#85), will ureq 2.x conflict in your env?

[RCasatta]

I think #102 is approximately solving this by avoiding the download feature and providing the executable