Trim trailing whitespace when extracting release notes

[mokagio]

I'm not sure if the behavior occurs in both iOS and Android implementations, but it is definitely present in the iOS version.

See https://github.com/Automattic/simplenote-ios/pull/1662/files#diff-8380791d5143cd014faa8d86802aa4a021ad745bf3b85cda46c87891e048e2c4