Update Gemfile to use `bundle config --local`

This pull request supports bundler feature to use local git repositories.

```
$ bundle config local.activerecord /path/to/rails
```
Although similar pull request #178 was closed Oracle enhanced adapter version 1.5 supports Rails 4.0 only I think we can support this feature.

For example ruby-oci8 master branch supports Ruby 2.1.0-preview1 in https://github.com/kubo/ruby-oci8/commit/80be58a0259b830db6a1f24ce0cb1020a42a5dd7 in the master branch. 
To test this commit we need to modify the Gemfile in Oracle enhanced adapter such as adding environment variables which shows the full path.

I prefer not to add any more environment variables to oracle enhanced adapter by making use of bundler standard features.