Bump version to 1.5.9.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124864123

RELEASENOTES.md

 # Release notes #
 # Release notes #
 
 
-### Current dev branch ###
+### r2.x ###
+
+ExoPlayer 2.x is a major rewrite that's still under development. We suggest
+following our [Medium page](https://medium.com/google-exoplayer) to learn more,
+as we work toward writing more complete documentation and the first 2.x release.
+
+### r1.5.9 ###
+
+* MP4: Fixed incorrect sniffing in some cases (#1523).
+* MP4: Improved file compatibility (#1567).
+* ID3: Support for TIT2 and APIC frames.
+* Fixed querying of platform decoders on some devices.
+* Misc bug fixes.
+
+### r1.5.8 ###
+
+* HLS: Fix handling of HTTP redirects.
+* Audio: Minor adjustment to improve A/V sync.
+* OGG: Support FLAC in OGG.
+* TTML: Support regions.
+* WAV/PCM: Support 8, 24 and 32-bit WAV and PCM audio.
+* Misc bug fixes and performance optimizations.
 
 
 ### r1.5.7 ###
 ### r1.5.7 ###