Change ureq into caret version.

[rajarshimaitra]

Relax ureq version requirement. Current ureq is at v2.5.0 so we should be able to use that if other crates have the requirement.

Motivated by failure observed in building bdk-cli with latest electrsd

$ cargo build
    Updating crates.io index
error: failed to select a version for `ureq`.
    ... required by package `bitcoind v0.28.0`
    ... which satisfies dependency `bitcoind = "^0.28.0"` of package `electrsd v0.22.0`
    ... which satisfies dependency `electrsd = "^0.22"` of package `bdk-cli v0.6.0 (/home/raj/github-repo/bdk-cli)`
versions that meet the requirements `^2.5.0` are: 2.5.0

all possible versions conflict with previously selected packages.

  previously selected package `ureq v2.2.0`
    ... which satisfies dependency `ureq = "~2.2.0"` of package `esplora-client v0.1.1`
    ... which satisfies dependency `esplora-client = "^0.1.1"` of package `bdk v0.23.0`
    ... which satisfies dependency `bdk = "^0.23"` of package `bdk-cli v0.6.0 (/home/raj/github-repo/bdk-cli)`

[coveralls]

Pull Request Test Coverage Report for Build 3356292949

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.894%

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Totals
Change from base Build 3313936566:	0.0%
Covered Lines:	699
Relevant Lines:	886

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💛 - Coveralls

[notmandatory]

This should be fixed in #24 so closing this one. :-)