[ruby] `require': no such file to load -- bindata (LoadError)

pc_liao@tw-pcliao1[~/code/git-tools/parse_index_file]{17:06}$ ruby parse_index_file.rb ../.git/index
parse_index_file.rb:10:in `require': no such file to load -- bindata (LoadError)
from parse_index_file.rb:10
pc_liao@tw-pcliao1[~/code/git-tools/parse_index_file]{17:06}$ gem install bindata
Fetching: bindata-1.4.5.gem (100%)
Successfully installed bindata-1.4.5
1 gem installed
Installing ri documentation for bindata-1.4.5...
Installing RDoc documentation for bindata-1.4.5...
pc_liao@tw-pcliao1[~/code/git-tools/parse_index_file]{17:08}$