[Java][Retrofit2] update retrofit2 dependencies

[wing328]

PR checklist

• Read the contribution guidelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

update retrofit2 dependencies

For play, we keep retrofit2 version to 2.1.0 due to the following errors:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project swagger-java-client: Compilation failure: Compilation failure:
[ERROR] /Users/williamcheng/Code/jan2017/swagger-codegen/samples/client/petstore/java/retrofit2-play24/src/main/java/io/swagger/client/Play24CallAdapterFactory.java:[17,23] wrong number of type arguments; required 2
[ERROR] /Users/williamcheng/Code/jan2017/swagger-codegen/samples/client/petstore/java/retrofit2-play24/src/main/java/io/swagger/client/Play24CallAdapterFactory.java:[44,24] wrong number of type arguments; required 2
[ERROR] /Users/williamcheng/Code/jan2017/swagger-codegen/samples/client/petstore/java/retrofit2-play24/src/main/java/io/swagger/client/Play24CallAdapterFactory.java:[52,59] wrong number of type arguments; required 2
[ERROR] /Users/williamcheng/Code/jan2017/swagger-codegen/samples/client/petstore/java/retrofit2-play24/src/main/java/io/swagger/client/Play24CallFactory.java:[82,12] io.swagger.client.Play24CallFactory.PlayWSCall is not abstract and does not override abstract method clone() in okhttp3.Call
[ERROR] /Users/williamcheng/Code/jan2017/swagger-codegen/samples/client/petstore/java/retrofit2-play24/src/main/java/io/swagger/client/Play24CallAdapterFactory.java:[60,9] method does not override or implement a method from a supertype
[ERROR] /Users/williamcheng/Code/jan2017/swagger-codegen/samples/client/petstore/java/retrofit2-play24/src/main/java/io/swagger/client/Play24CallAdapterFactory.java:[65,9] method does not override or implement a method from a supertype