Drop the 'r' from release version.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175543465

RELEASENOTES.md

 # Release notes #
 
-### r2.6.0 ###
+### 2.6.0 ###
 
+* Removed "r" prefix from versions. This release is "2.6.0", not "r2.6.0".
 * New `Player.DefaultEventListener` abstract class can be extended to avoid
   having to implement all methods defined by `Player.EventListener`.
 * Added a reason to `EventListener.onPositionDiscontinuity`

constants.gradle

@@ -28,7 +28,7 @@ project.ext {
     junitVersion = '4.12'
     truthVersion = '0.35'
     robolectricVersion = '3.4.2'
-    releaseVersion = 'r2.6.0'
+    releaseVersion = '2.6.0'
     modulePrefix = ':'
     if (gradle.ext.has('exoplayerModulePrefix')) {
         modulePrefix += gradle.ext.exoplayerModulePrefix