Second class letter fix

Fixes: https://github.com/meanbee/royalmail/issues/18

Cleanest way to fix this and to maintain backwards compatibility was to create two new rates "Second Class Letter" and "Second Class Large Letter". 

I noticed International Economy was broken since this commit: https://github.com/meanbee/royalmail/pull/13/files#diff-8125a79c8c56d79d9ebe841ccca8a885L24, it required a negative check to pass as you cannot send a parcel on international economy within gb.

PHPUnit tests were broken based on updated pricing. This has been resolved, and new tests introduced to cover the two new rates.
Should this be Meanbee_Royalmail_Helper_Data::WORLD_ZONE_GB as well?