Fixes atomic operations on read-only attributes

https://github.com/mongoid/mongoid/issues/3612
more specs for bit and inc readonly
Use moped 2.0.0.rc1 on gemspec
Add check for mongodb_version on bitwise tests

[fixes #3609]
Merge pull request #3613 from fred/readonly-atomic

Don't allow atomic operations on read-only attributes
Add changelog for #3613

Also small cleanup