fix: Change trust limit

[aidan46-crypto]

Issue

There is no way to remove a trust line.

Fix

Update the change trust operation limit check to allow a 0 limit, to be able to remove trust lines. This fix is trivial, update the limit check in the ChangeTrustOprationBuilder build() function.

// Change `<=` to `<` to be able to remove trust lines
if limit.to_i64() <= 0 { 
    return Err(Error::InvalidOperation(
        "change trust limit must be positive".to_string(),
    ));
}

[bjorn]

@aidan46-crypto Would you update this PR with a slightly adjusted error message, and sign-off your commit so CI will pass (it needs to also be rebased after #6 is merged)? I am not able to change the PR myself (maybe you could enable me to push to your master for this PR, but I'm not sure if GitHub allows this). If not, I would just manually re-apply the fix.

[bjorn]

Since we can't move forward with this PR without @aidan46-crypto, I've opened #11 as a replacement.