Deprecate #connection in favour of accessing it via the class

see rails/rails#9371


This pull request suppresses following `DEPRECATE WARNING`.
```ruby
$ rspec ./spec/active_record/connection_adapters/oracle_enhanced_data_types_spec.rb:885
==> Running specs with MRI version 2.0.0
==> Running specs with Rails version 4.0-master
Run options: include {:locations=>{"./spec/active_record/connection_adapters/oracle_enhanced_data_types_spec.rb"=>[885]}}
..DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:123)
DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:128)
..DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:123)
reword 3818d6f Initial
Rebasing (1/1)
Deprecate #connection in favour of accessing it via the class
DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:128)
.DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:123)
DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:128)
.DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:123)
DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:128)
.DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:123)
DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:128)
.DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:123)
DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:128)
.DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:123)
DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:128)
.DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:123)
DEPRECATION WARNING: #connection is deprecated in favour of accessing it via the class. (called from enhanced_write_lobs at /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:128)
.

Finished in 0.55475 seconds
11 examples, 0 failures
$
```