BUG: Add double-conversion license.

So all third party libraries included with ITK have clear licenses.

Change-Id: If1b57bfc3ac3cc24235daffbff0f3273ae71badc
ENH: Add check for LICENSE change on double-conversion update.

Change-Id: I4f6565a11f67ee1f229ab4bd6777e671539507a4
BUG: Fix removal of temporary double-conversion repository.

Change-Id: I5177afc02f6b7e1ddb4f329f7cf9514022b70d9c
Merge topic 'double-conversion-license'

91280bb BUG: Fix removal of temporary double-conversion repository.
4e20cc2 ENH: Add check for LICENSE change on double-conversion update.
c9971ad BUG: Add double-conversion license.