Supporting Mac OS system Ruby.

The system Ruby that ships with Mac OS does not support multiple `-r`
options. The changes the behavior so we only pass one `-r` option and
require the pre-commit gem as part of the command passed to `-e`.
Integration tests do not depend on implementation.

(hopefully)

We were counting on an implementation detail of the pre commit hook
template to run integration tests on ci servers. The hook used to
require the pre-commit library using the `-r` Ruby option.
Unfortunately old versions of Ruby do not support passing the `-r`
option multiple times. When we changed the hook to only pass one `-r`
option the integration tests started failing.

In order to get around this, we're hooking into a constant called
`OPTIONS`. I'm not sure if this is actually any better, as we're
still counting on the fact that the `OPTIONS` constant is defined in
the pre commit hook template.

To help alleviate some pain in the future, we have a nicer test
failure message if the `OPTIONS` constant is missing.
Use Bundler.root to find out where we ran tests from.
Merge pull request #66 from jish/josh/system_ruby_support

Josh/system ruby support