astyanax 3.9.0 version will support for Cassandra 2.1.x. But you need to add some other supported dependencies.
I'm using Cassandra 2.1.4 and these are all added dependencies.
<dependency>
<groupId>com.netflix.astyanax</groupId>
<artifactId>astyanax</artifactId>
<version>3.9.0</version>
</dependency>
<dependency>
<groupId>com.netflix.astyanax</groupId>
<artifactId>astyanax-cassandra</artifactId>
<version>3.9.0</version>
</dependency>
<dependency>
<groupId>com.netflix.astyanax</groupId>
<artifactId>astyanax-core</artifactId>
<version>3.9.0</version>
</dependency>
<dependency>
<groupId>com.netflix.astyanax</groupId>
<artifactId>astyanax-cql</artifactId>
<version>3.9.0</version>
</dependency>
<dependency>
<groupId>com.netflix.astyanax</groupId>
<artifactId>astyanax-thrift</artifactId>
<version>3.9.0</version>
</dependency>
<dependency>
<groupId>com.netflix.astyanax</groupId>
<artifactId>astyanax-entity-mapper</artifactId>
<version>3.9.0</version>
</dependency>
<dependency>
<groupId>org.apache.cassandra</groupId>
<artifactId>cassandra-all</artifactId>
<version>3.2.1</version>
</dependency>
<dependency>
<groupId>com.netflix.astyanax</groupId>
<artifactId>astyanax-recipes</artifactId>
<version>3.9.0</version>
</dependency>