minor: remove phantom puts
minor: fixing misleading code comment in bson_ext
RUBY-475 refactor read preference logic

Move read_pool into Mongo::ReadPreference
Add #read_preference to produce read preference hash
RUBY-475 pin/unpin logic added to cursor
minor: moving away from .rvmrc, adding gitignore for .ruby-version
minor: merge should be called with parens here
RUBY-537 cleaning up how strict is used, deprecating strict