Bump retrofit-version from 2.3.0 to 2.6.2

[dependabot]

Bumps retrofit-version from 2.3.0 to 2.6.2.

Updates converter-gson from 2.3.0 to 2.6.2

Updates retrofit from 2.3.0 to 2.6.2

Changelog

Sourced from retrofit's changelog.

Version 2.6.2 (2019-09-23)

• Fix: Avoid IOExceptions being wrapped in UndeclaredThrowableException in rare cases when using
  Response<..> as a return type for Kotlin 'suspend' functions.

Version 2.6.1 (2019-07-31)

• Fix: Avoid IOExceptions being wrapped in UndeclaredThrowableException in rare cases.
• Fix: Include no-content ResponseBody for responses created by Response.error.
• Fix: Update embedded R8/ProGuard rules to not warn about nested classes used for Kotlin extensions.

Version 2.6.0 (2019-06-05)

• New: Support suspend modifier on functions for Kotlin! This allows you to express the asynchrony of HTTP requests
  in an idiomatic fashion for the language.

  @GET("users/{id}")
  suspend fun user(@Path("id") id: Long): User

  Behind the scenes this behaves as if defined as fun user(...): Call<User> and then invoked with Call.enqueue.
  You can also return Response<User> for access to the response metadata.

  Currently this integration only supports non-null response body types. Follow
  issue 3075 for nullable type support.

• New: @Tag parameter annotation for setting tags on the underlying OkHttp Request object. These can be read
  in CallAdapters or OkHttp Interceptors for tracing, analytics, varying behavior, and more.

• New: @SkipCallbackExecutor method annotation will result in your Call invoking its Callback on the
  background thread on which the HTTP call was made.

• New: Support OkHttp's Headers type for @HeaderMap parameters.

• New: Add Retrofit.Builder.baseUrl(URL) overload.

• Fix: Add embedded R8/ProGuard rule which retains Retrofit interfaces (while still allowing obfuscation). This
  is needed because R8 running in 'full mode' (i.e., not in ProGuard-compatibility mode) will see that there are
  no subtypes of these interfaces and rewrite any code which references instances to null.

• Fix: Mark HttpException.response() as @Nullable as serializing the exception does not retain this instance.

• Fix: Fatal errors (such as stack overflows, out of memory, etc.) now propagate to the OkHttp Dispatcher thread
  on which they are running.

• Fix: Ensure JAX-B converter closes the response body when an exception is thrown during deserialization.

• Fix: Ignore static methods when performing eager validation of interface methods.

... (truncated)

Commits

• See full diff in compare view

Updates converter-scalars from 2.3.0 to 2.6.2

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