build: temporarily change "dockertest-server" dependency

Issuers support was tested with the latest "1.15.1" vault version

revert when the following PR will be merged:
jmgilman/dockertest-server#12

tests/common.rs

@@ -18,7 +18,7 @@ use vaultrs::{
 };
 
 pub const PORT: u32 = 8300;
-pub const VERSION: &str = "1.10.3";
+pub const VERSION: &str = "1.15.1";
 pub const NGINX_PORT: u32 = 8888;
 pub const NGINX_VERSION: &str = "1.21";
 pub const LOCALSTACK_VERSION: &str = "2.0.2";