fix bundle install for ruby 1.8.7

Bundler gets removed from the `$LOAD_PATH` for some reason in ruby 1.8.7.
This means we shouldn't try to require something from bundler here.

```
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native
extension.

        /home/hone/.rvm/rubies/ruby-1.8.7-p371/bin/ruby extconf.rb
/home/hone/.rvm/rubies/ruby-1.8.7-p371/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:36:in
`gem_original_require': no such file to load -- bundler/psyched_yaml
(LoadError)
        from
/home/hone/.rvm/rubies/ruby-1.8.7-p371/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:36:in
`require'
        from
/home/hone/Projects/heroku_work/heroku-buildpack-ruby/vendor/syck_hack.rb:15
```