travis: use the latest Ruby 2.1
travis: run benchmark script for #16 on Travis CI

It will be reverted soon.
Add missing include path
travis: add debug print
travis: use bundle exec
Remove needless code
Remove needless script
Fix a memory leak of Groonga::Table#column

Groonga::Table#columns doesn't set name to retrieved columns. So cache
system in Groonga::Table#column is broken. It causes a memory leak.

GitHub: fix #16

Reported by rutice. Thanks!!!
grndump: don't use needless "--key_type" for table_create

Reported by Genki Takiuchi. Thanks!!!
document: add missing options
Add sources to index column inspect
Use buffer on stack
Enable with_section flag for multiple column index
Follow inspect output change of index column
Fix implement location
Use "nil" for nonexistent
Update the latest Groonga version
Add 4.0.2 entry
Enable with_position flag by default for a table that has the default tokenizer
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