Changed 'version' filename to honor reserved keyword. (#5434)

* Changed 'version' filename to honor cmake reserved keyword.

Changed 'version' filename to isis_version.txt. 'version' is a reserved
keyword in cmake.  Additionally, the extension should prevent this file
from being compiled as source, and eliminate the recompilation error.

* Updated changelog

* Reverted accidental change to version

* Updated version file name