`bundle install` fails when run inside directory with spaces

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (either `gem update github-pages` or `bundle update github-pages`)

### This issue affects

- [ ] The site generated by GitHub Pages
- [x] Building sites locally

### What did you do (e.g., steps to reproduce)

```
mkdir "Test Directory"
cd "Test Directory"
echo "source 'https://rubygems.org'" > Gemfile
echo "gem 'github-pages', group: :jekyll_plugins" >> Gemfile
bundle install --path vendor/bundle
```

### What did you expect to happen?

Everything to install correctly.

### What happened instead?

The `configure` script for ffi doesn't quote `$srcdir`, which breaks building:

```
Fetching gem metadata from https://rubygems.org/...........
Fetching version metadata from https://rubygems.org/..
Fetching dependency metadata from https://rubygems.org/.
Resolving dependencies...
[...]
Installing ffi 1.9.18 with native extensions
[...]
/Users/rmccue/Test Directory/vendor/bundle/ruby/2.0.0/gems/ffi-1.9.18/ext/ffi_c/libffi/configure: line 2659: /Users/rmccue/Test: No such file or
directory
```