Bump libraryVersion.retrofit2 from 2.4.0 to 2.8.1

[dependabot-preview]

Bumps libraryVersion.retrofit2 from 2.4.0 to 2.8.1.
Updates retrofit from 2.4.0 to 2.8.1

Changelog

Sourced from retrofit's changelog.

Version 2.8.1 (2020-03-25)

• Fix: Do not access MethodHandles.Lookup on Android API 24 and 25. The class is only available on Android API 26 and higher.

Version 2.8.0 (2020-03-23)

• New: Add Call.timeout() which returns the okio.Timeout of the full call.
• Fix: Change Call.awaitResponse() to accept a nullable response type.
• Fix: Support default methods on Java 14+. We had been working around a bug in earlier versions of Java. That bug was fixed in Java 14, and the fix broke our workaround.

Version 2.7.2 (2020-02-24)

• Fix: Update to OkHttp 3.14.7 for compatibility with Android R (API 30).

Version 2.7.1 (2020-01-02)

• Fix: Support 'suspend' functions in services interfaces when using 'retrofit-mock' artifact.

Version 2.7.0 (2019-12-09)

This release changes the minimum requirements to Java 8+ or Android 5+. See this blog post for more information on the change.

• New: Upgrade to OkHttp 3.14.4. Please see its changelog for 3.x.
• Fix: Allow service interfaces to extend other interfaces.
• Fix: Ensure a non-null body is returned by Response.error.

Version 2.6.4 (2020-01-02)

• Fix: Support 'suspend' functions in services interfaces when using 'retrofit-mock' artifact.

Version 2.6.3 (2019-12-09)

• Fix: Change mechanism for avoiding UndeclaredThrowableException in rare cases from using yield an explicit dispatch which ensures that it will work even on dispatchers which do not support yielding.

... (truncated)

Commits

• 7fa4b3e [maven-release-plugin] prepare release parent-2.8.1
• 704c484 Prepare version 2.8.1
• 49aa9fe Merge pull request #3344 from square/jakew/get-a-handle/2020-03-24
• 5850333 Don't attempt to resolve Lookup ctor on API 24/25
• 5876f05 Update changelog for 2.8.0
• e781a69 [maven-release-plugin] prepare for next development iteration
• 2cde2ae [maven-release-plugin] prepare release parent-2.8.0
• 955dde3 Add link to okhttp & okio proguard rules
• bfb5cd3 Merge pull request #3340 from square/zachklipp/awaitresponse-allownulls
• 9106005 Allow awaitResponse() to be called with nullable type parameter.
• Additional commits viewable in compare view

Updates adapter-rxjava2 from 2.4.0 to 2.8.1

Updates converter-gson from 2.4.0 to 2.8.1

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[dependabot-preview]

Superseded by #87.