PROTON-445: Perl install now honors CMAKE_INSTALL_PREFIX

The language bindings will now honor this prefix, both the default
value and a user-specified value.

Developers can override it, though, by specifying PERL_INSTALL_PREFIX
at the command line as well. This will allow for installing the Perl
bindings to a different directory if so desired.

Also updated the installation pieces in CMake to simplify it.
PROTON-464: Install Ruby bindings to install prefix if specified.

If the user provides a CMAKE_INSTALL_PREFIX on the command line, then
that directory is used to install the Ruby language bindings. If no
install prefix is specified then the Ruby bindings honor the directory
specified by the Ruby VM itself.