Fix "undefined local variable or method `adapter'"

NameError: undefined local variable or method `adapter' for #<DataMapper::Adapters::SqliteAdapter:0x88caa5c>
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/data_mapper/truncation.rb:15:in `block in truncate_tables'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/data_mapper/truncation.rb:14:in `each'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/data_mapper/truncation.rb:14:in `truncate_tables'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/data_mapper/truncation.rb:192:in `block in clean'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/data_mapper/truncation.rb:110:in `disable_referential_integrity'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/data_mapper/truncation.rb:191:in `clean'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/base.rb:86:in `clean'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/configuration.rb:79:in `block in clean'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/configuration.rb:79:in `each'
    /home/pirj/.rvm/gems/ruby-2.1.2@g5/gems/database_cleaner-1.3.0/lib/database_cleaner/configuration.rb:79:in `clean'