Improve robustness of QML plugin loading

Use a variable that expands to the latest plugin version instead of
hard-coding the string.

Change-Id: I8f7864e4ead85487943ed2c4dc2ae2584005c1c9
Reviewed-by: Paolo Angelelli <paolo.angelelli@theqtcompany.com>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>