Align retrofit2 versions to ensure dependency convergence

[mnk]

When depending on com.segment.analytics.java:analytics and using Maven DependencyConvergence enforcer, build will fail with:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.5.0:enforce (dependencies) on project commons-segment:
[ERROR] Rule 16: org.apache.maven.enforcer.rules.dependency.DependencyConvergence failed with message:
[ERROR] Failed while enforcing releasability.
[ERROR]
[ERROR] Dependency convergence error for com.squareup.retrofit2:retrofit:jar:2.11.0 paths to dependency are:
[ERROR]   +-com.segment.analytics.java:analytics:jar:3.5.2:compile
[ERROR]     +-com.segment.analytics.java:analytics-core:jar:3.5.2:compile
[ERROR]       +-com.squareup.retrofit2:retrofit:jar:2.11.0:compile
[ERROR] and
[ERROR]   +-com.segment.analytics.java:analytics:jar:3.5.2:compile
[ERROR]     +-com.squareup.retrofit2:converter-gson:jar:2.9.0:compile
[ERROR]       +-com.squareup.retrofit2:retrofit:jar:2.9.0:compile
[ERROR] and
[ERROR]   +-com.segment.analytics.java:analytics:jar:3.5.2:compile
[ERROR]     +-com.squareup.retrofit2:retrofit-mock:jar:2.11.0:compile
[ERROR]       +-com.squareup.retrofit2:retrofit:jar:2.11.0:compile