Deprecate XingApi's resource function

See changelog for details

Bump of moshi minsdk targetsdk kotlin and supportLib version

• Update moshi to 1.8.0
• Update minSdk to 21
• Update targetSdk to 28
• Update supportLib to 28.0.0
• Update build tools to 28.0.3
• Update kotlin version to 1.3.20

Bump version 3.0.6 (#243)

* Update readme

This reverts commit d3881391a7ca9d61f3d274c5bc08116383597143.

* Bump version to 3.0.6

This reverts commit 666a5467e4e78482967241ee6b69943aedd93427.

* Correct version code
The [docs](https://developer.android.com/studio/publish/versioning) tell that the higher the version code, the newer the version:
`monotonically increase the value with each release`

3.0.1

Bump to 3.0.1 (#235)

3.0.0

We do not support RxJava1 anymore.
Removed Rx1 dependencies and calls (#232)