chore(deps): bump retrofit from 2.5.0 to 2.6.0

[dependabot-preview]

Bumps retrofit from 2.5.0 to 2.6.0.

Changelog

Sourced from retrofit's changelog.

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") long id): 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.

• Fix: Ensure that calling source() twice on the ResponseBody passed to a Converter always returns the same
  instance. Prior to the fix, intermediate buffering would cause response data to be lost.

Commits

• b8d593c [maven-release-plugin] prepare release parent-2.6.0
• aeb7b6c Prepare next development version
• defefd6 Merge pull request #3114 from joelhandwell/patch-1
• c04ebd2 Add JAXB Converter in Document
• ce6e0d1 URL overload for Retrofit.Builder#baseUrl
• 81a6193 Merge pull request #3070 from NightlyNexus/eric.2019-04-09.headers-parameter
• a7755ea Support okhttp3.Headers for HeadersMap parameters.
• fc14b32 Merge pull request #3065 from square/jakew/versions/2019-04-03
• c7df6b8 Next version is 2.6.0
• faa62a8 Merge pull request #3064 from ojh102/bump_gson
• Additional commits viewable in compare view

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