switch to Class::Load for loading classes

Class::MOP::load_class has been deprecated, see
https://metacpan.org/pod/release/ETHER/Moose-2.1106-TRIAL/lib/Moose/Manual/Delta.pod#pod2.1200

This generates warnings when using Net::Google::DataAPI:
```
Class::MOP::load_class is deprecated at /usr/local/lib/perl/5.14.2/Class/MOP.pm line 68.
    Class::MOP::load_class("Net::Google::Spreadsheets::Spreadsheet") called at /usr/local/share/perl/5.14.2/Net/Google/DataAPI.pm line 108
    Net::Google::Spreadsheets::spreadsheets(Net::Google::Spreadsheets=HASH(0x48a23e0), HASH(0x3ae5438)) called at /usr/local/share/perl/5.14.2/Net/Google/Spreadsheets.pm line 57
    Class::MOP::Class:::around(CODE(0x561b810), Net::Google::Spreadsheets=HASH(0x48a23e0), HASH(0x3ae5438)) called at /usr/local/lib/perl/5.14.2/Class/MOP/Method/Wrapped.pm line 157
    Class::MOP::Method::Wrapped::__ANON__(Net::Google::Spreadsheets=HASH(0x48a23e0), HASH(0x3ae5438)) called at /usr/local/lib/perl/5.14.2/Class/MOP/Method/Wrapped.pm line 86
    Net::Google::Spreadsheets::spreadsheets(Net::Google::Spreadsheets=HASH(0x48a23e0), HASH(0x3ae5438)) called at /usr/local/share/perl/5.14.2/Net/Google/DataAPI.pm line 132
```