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.