sub assert_installation -> sub assert_known_installation

fix for this problem:

perlbrew alias create perl-5.14.4 ops
Can't locate object method "assert_known_installation" via package "App::perlbrew" at /loader/0xd155d0/App/perlbrew.pm line 42.

Seemed to me that the subroutine was defined as 'assert_installation' but used as 'assert_known_installation'.  So I changed
the subroutine name to 'assert_known_installation' since it was called that multiple times.