AUI-1480 aui-dropdown HISTORY.md

Subtask of AUI-1458: Update HISTORY.md to reflect source code changes.
Titles for each entry are from the corresponding jira ticket.
aui-dropdown did not have commit ac999be3361683ccb7a7062b5e347a5e3199dc38
which is what the other HISTORY.md files are starting with. Instead
there were two jira tickets for the commits made to this component,
so those are recorded
AUI-1480 aui-dropdown HISTORY.md

Added the aui version number for the entries in the HISTORY.md filed
AUI-1480 - Source formatting
AUI-1480 Source formatting