Irs beta 48 is now available

Changes:
- Updated the underlying vmime library to its current latest and greatest state from github
- Iris (and vmime) are now built using GCC 8
- Fixed an input parsing bug that cound in some cases lead to an infinite loop when preparing a message to be displayed
- Fixed an issue with HTML views not rendering anymore after closing a message window
- Fixed: The message loader thread used in Load Message didn't have enough stack, leading to a disastrous end when opening emails from file that had a more complex structure (attachments)