Use `execute_batch2` rather than `execute_batch` to fix performance regression for fixture loading

d8d6bd5 makes fixture loading to bulk statements by using
`execute_batch` for sqlite3 adapter. But `execute_batch` is slower and
it caused the performance regression for fixture loading.

In sqlite3 1.4.0, it have new batch method `execute_batch2`. I've
confirmed `execute_batch2` is extremely faster than `execute_batch`.
So I think it is worth to upgrade sqlite3 to 1.4.0 to use that method.

Before:

```
% ARCONN=sqlite3 bundle exec ruby -w -Itest test/cases/associations/eager_test.rb -n test_eager_loading_too_may_ids
Using sqlite3
Run options: -n test_eager_loading_too_may_ids --seed 35790

# Running:

.

Finished in 202.437406s, 0.0049 runs/s, 0.0049 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips
ARCONN=sqlite3 bundle exec ruby -w -Itest  -n test_eager_loading_too_may_ids  142.57s user 60.83s system 98% cpu 3:27.08 total
```

After:

```
% ARCONN=sqlite3 bundle exec ruby -w -Itest test/cases/associations/eager_test.rb -n test_eager_loading_too_may_ids
Using sqlite3
Run options: -n test_eager_loading_too_may_ids --seed 16649

# Running:

.

Finished in 8.471032s, 0.1180 runs/s, 0.1180 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips
ARCONN=sqlite3 bundle exec ruby -w -Itest  -n test_eager_loading_too_may_ids  10.71s user 1.36s system 95% cpu 12.672 total
```

1 files changed, 1 insertions, 1 deletions

diff --git a/Gemfile b/Gemfile
index e80691bce5..122baa4c33 100644
--- a/Gemfile
+++ b/Gemfile
@@ -118,7 +118,7 @@ platforms :ruby, :mswin, :mswin64, :mingw, :x64_mingw do
   gem "racc", ">=1.4.6", require: false
 
   # Active Record.
-  gem "sqlite3", "~> 1.3", ">= 1.3.6"
+  gem "sqlite3", "~> 1.4"
 
   group :db do
     gem "pg", ">= 0.18.0"