Bump retrofit from 1.6.1 to 1.9.0 in /fineract-provider

Bumps [retrofit](https://github.com/square/retrofit) from 1.6.1 to 1.9.0.
- [Release notes](https://github.com/square/retrofit/releases)
- [Changelog](https://github.com/square/retrofit/blob/master/CHANGELOG.md)
- [Commits](https://github.com/square/retrofit/compare/parent-1.6.1...parent-1.9.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

fineract-provider/build.gradle

@@ -83,7 +83,7 @@ dependencyManagement {
 
         dependency 'org.springframework.security.oauth:spring-security-oauth2:2.3.6.RELEASE'
         dependency "org.apache.openjpa:openjpa:3.1.1" // when upgrading, also change OpenJPA version repeated above in buildscript!
-        dependency 'com.squareup.retrofit:retrofit:1.6.1'
+        dependency 'com.squareup.retrofit:retrofit:1.9.0'
         dependency 'com.squareup.okhttp:okhttp:2.0.0'
         dependency 'com.squareup.okhttp:okhttp-urlconnection:2.0.0'
         dependency 'com.google.code.gson:gson:+'