Show original error message when loading ruby-oci8 library fails

While I had the ruby-oci8 library already installed, I was puzzled by this error:

> ERROR: ActiveRecord oracle_enhanced adapter could not load ruby-oci8 library. Please install ruby-oci8 gem.

After revealing the original load error received, I discovered that the original message was:

> LoadError: libaio.so.1: cannot open shared object file: No such file or directory

So I got rid of this (on Travis CI), by installing `libaio1` via `sudo apt-get install libaio1`