doc cross-compile: add Linux Users section
doc cross-compile: add description for vagrant box users
doc cross-compile: rewrite section title

`apt` has only Debian GNU/Linux like environment.
travis: add Ruby 2.2
test: add time zone check for debug
Revert "test: add time zone check for debug"

This reverts commit 7e2e3603fabbcd28ad54a744d105e6900c3664c4.

test_time_zone for only travis CI debug.
This test should be reverted.
test: specify TimeZone

```ruby
ENV["TZ"] = "Japan"
```

ref: https://bugs.ruby-lang.org/issues/9652
ref: http://svn.ruby-lang.org/cgi-bin/viewvc.cgi?revision=45364&view=revision
test plugin: use Power Assert

This test code becomes more natural rubyish code.
Let's start using Power Assert!
gemspec: bump up Test::Unit dependency

Power Assert has been introduced since Test::Unit 3.0.0
Fix a typo

differ nt ->
different
      ^
doc cross-compile: update Ruby 2.1 series description

2.1.2 ->
2.1.3
    ^
windows: use Ruby 2.1.3
doc cross-compile: update Ruby 2.0.0 cross compile description

Ruby 2.0.0-p576 has been released!!
windows: use latest Ruby 2.0 for cross compiling
Add Groonga::DoubleArrayTrie#update

This is a low level API for `grn_table_update_by_id()` and
`grn_table_update()`.

[GitHub #22] Bind `grn_table_update_by_id()` and `grn_table_update()`

TODO: add an easy-to-use API (e.g.: Groonga::Record#rename).
Use my name
Add missing "static"
Remove needless `NIL_P` check

Because `rb_grn_scan_options()` accepts `nil` as `options`.
Unindent
extconf: add have_func checks for `rb_sym2str()` and `rb_str2sym()`