spec/: Refactored and extended.
lib/libcdb.rb (load{,_file}): New methods.
lib/libcdb.rb ({,print_}stats): New methods.
ext/libcdb/ruby_cdb_writer.c (rcdb_writer_put_value): Ensure key is a string.
ext/libcdb/ruby_cdb_reader.c (rcdb_reader_each_dump): Ensure key is a string.
ext/libcdb/ruby_cdb_reader.c: Added encoding support.
.travis.yml: Dropped Rubinius support ("undefined symbol: rb_enc_default_external"). (edc09f6)