Upgrade to use R.NET 1.5.15

Upgrade to use R.NET 1.5.15 to prevent possible issues noticed after a memory GC fix was added for Deedle. May or may not be an issue in different scenarios, but probably a good idea to upgrade. See https://rclr.codeplex.com/workitem/31 and https://rdotnet.codeplex.com/workitem/120. @dcharbon may want to look at the commit https://rdotnet.codeplex.com/SourceControl/changeset/1212c26806be to assess whether this fix has the potential to re-break Deedle.