readme: update connector version to 1.9.4

Users often copy-paste this hunk for Maven and receive the old connector
version with known problems. It seems we should update README after each
release to cleanly state that we recommend the last version to use.

Follows up b1e4b59 ('readme: add a link
to releases page').

README.md

@@ -31,7 +31,7 @@ To get the Java connector for Tarantool 1.6.9, visit
 <dependency>
   <groupId>org.tarantool</groupId>
   <artifactId>connector</artifactId>
-  <version>1.9.2</version>
+  <version>1.9.4</version>
 </dependency>
 ```