Merge pull request #25 from JPMoresmau/master

Relax dependency bounds, do not require IDs in JSON + cardId fix
Show card's currency when creating payin on test app.
Bump version to 1.9.2.
The BIC is now optional [1].

[1] http://docs.mangopay.com/release-poulet/
Bump versions to 1.10 due to BIC change.
Fix account test, add test without BIC.
Strip absent (Nothing) fields.  All tests pass.

This was causing problems when creating bank accounts with IBAN but no
BIC.  MangoPay doesn't accept a null BIC, instead the BIC field needs to
be omitted altogether.

Although not necessary in order to make the tests pass, I've also
changed all other objects to strip absent fields.  This is in order to
be consistent across the whole library.
Documentation