Use maven publish plugin for central publishing

[ZacSweers]

This simplifies setup using the gradle maven publish plugin for publishing to central. This removes jcenter publishing, but publishing just to central covers jcenter since it proxies it anyway.

https://github.com/vanniktech/gradle-maven-publish-plugin

This requires added local gradle properties (recommended to put them in your ~/.gradle/gradle.properties) for signing and auth, described here: https://github.com/vanniktech/gradle-maven-publish-plugin#where-to-upload-to

Most significantly, this automatically handles publishing the gradle plugin marker artifact as well so that it can be used directly in plugins {...}

This also makes snapshots development (./gradlew uploadArchives when the version ends in -SNAPSHOT) and local development ./gradlew install) easier

Resolves #267

[ZacSweers]

This is... an interesting error

* What went wrong:
A problem occurred configuring root project 'app'.
> Could not resolve all artifacts for configuration ':classpath'.
   > Could not find com.bugsnag:bugsnag-android-gradle-plugin:9000.0.0-test.
     Searched in the following locations:
       - file:/home/travis/.m2/repository/com/bugsnag/bugsnag-android-gradle-plugin/9000.0.0-test/bugsnag-android-gradle-plugin-9000.0.0-test.pom
       - file:/home/travis/.m2/repository/com/bugsnag/bugsnag-android-gradle-plugin/9000.0.0-test/bugsnag-android-gradle-plugin-9000.0.0-test.jar
       - https://dl.google.com/dl/android/maven2/com/bugsnag/bugsnag-android-gradle-plugin/9000.0.0-test/bugsnag-android-gradle-plugin-9000.0.0-test.pom
       - https://dl.google.com/dl/android/maven2/com/bugsnag/bugsnag-android-gradle-plugin/9000.0.0-test/bugsnag-android-gradle-plugin-9000.0.0-test.jar
       - https://jcenter.bintray.com/com/bugsnag/bugsnag-android-gradle-plugin/9000.0.0-test/bugsnag-android-gradle-plugin-9000.0.0-test.pom
       - https://jcenter.bintray.com/com/bugsnag/bugsnag-android-gradle-plugin/9000.0.0-test/bugsnag-android-gradle-plugin-9000.0.0-test.jar
     Required by:
         project :

[fractalwrench]

Changes look good, just one query about whether it's still possible to publish to Bintray. CI also appears to be broken, potentially it might be the case of setting this flag to disable signing on CI: vanniktech/gradle-maven-publish-plugin#114

[ZacSweers]

Looks like it's working now 👍 . Would be worth exploring enabling snapshots btw, makes it easy for people to try new versions. This plugin handles it well too as long at the version ends in -SNAPSHOT