Fix "Libcaf" casing issue in FindLibcaf.cmake.

find_package_handle_standard_args() expects the first argument to be the
name of the Find-module in original casing.  If it's not, it may fail to
do things like abort with a FATAL_ERROR message if REQUIRED was supplied
to the original find_package().
Merge pull request #161 from jsiwek/master

Fix "Libcaf" casing issue in FindLibcaf.cmake.