Use backticks in PG rake:scheme:load task

Since upgrading to Rails 4, we have been seeing a lot of noise whenever we run our specs:

```
$ % rake spec             
SET
SET
SET
SET
SET
SET
CREATE EXTENSION
COMMENT
CREATE EXTENSION
COMMENT
SET
SET
SET
CREATE TABLE
CREATE SEQUENCE
ALTER SEQUENCE
CREATE TABLE
CREATE SEQUENCE
ALTER SEQUENCE
CREATE TABLE
CREATE SEQUENCE
ALTER SEQUENCE
...
```

After some investigation today it seems to be caused by [this change](https://github.com/rails/rails/commit/19911959d9a8fa5341f61b028cb489e6ecc2c339) that effectively switched from using backticks to `Kernel.system`.

I wasn't able to figure out how to configure `Kernel.system` to behave like the backtick method, but changing it back fixes the problem.