Remove update check from about box.

The about box should not conduct its own update check. Doing so creates
the possibility of two simultaneous updates occurring, and the update
download file locking.

This commit also removes two behaviors, ESC to close, clicking on
version number to install update,  which were not thread-safe.
Merge branch 'RC0.7.2_master' into RC0.7.2_Revit2015
Merge branch 'RC0.7.2_Revit2015' of https://github.com/DynamoDS/Dynamo into RC0.7.2_Revit2015