install roundcube from our own mirror, hosted in Josh's AWS S3 account, because sourceforge is down all the time

fixes #750, see #701, see #370

CHANGELOG.md

@@ -17,6 +17,7 @@ Setup:
 
 * A swap file is now created if system memory is less than 2GB, 5GB of free disk space is available, and if no swap file yet exists.
 * Setup dialogs did not appear correctly when connecting to SSH using Putty on Windows.
+* We now install Roundcube from our own mirror because Sourceforge's downloads experience frequent intermittant unavailability.
 
 v0.17b (March 1, 2016)
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