Specify exact BSON version dependency

Since the mongo and bson gems are versioned together, it should follow
that mongo depends on a specific version of bson. Otherwise, client
libraries that require a specific version of mongo, expecting the same
version of bson to be installed may get a different version. This is
particularly painful when a client library specifies a dependency on
both mongo and bson_ext using the same version number, which may not be
the latest, and the dependency from the mongo gem will actually install
a different version of bson than bson_ext.

mongo.gemspec

@@ -30,5 +30,5 @@ Gem::Specification.new do |s|
   s.email = 'mongodb-dev@googlegroups.com'
   s.homepage = 'http://www.mongodb.org'
 
-  s.add_dependency(%q<bson>, [">= #{Mongo::VERSION}"])
+  s.add_dependency('bson', Mongo::VERSION)
 end