Fix test bootstrap autoloader

The previous `require_once` statement returns `true` when it loads the
autoload file rather than an instance of `ClassLoader`. Changing the
statement to `require` returns the instance needed to add the Monolog
test directory.
Merge pull request #232 from adlawson/fix-bootstrap

Small fix to test bootstrap
Add PHPUnit to dev dependencies

The easier it is for people to contribute, the better. Relying on a
global install of PHPUnit via PEAR doesn't sit well with some
developers. As other dev dependencies are installed via composer, why
not add PHPUnit itself to the list?
Merge pull request #233 from adlawson/dep-phpunit

Add PHPUnit to dev dependencies