allow bundler-audit to switch directory to ruby project path

[dstull]

currently when running bundler-audit, it must be ran inside the directory where .bundle directory and the Gemfile.lock file exists - see this issue for enhancement on that package: rubysec/bundler-audit#178 and code which is currently unreachable in that gem from the command line where it is set on the PWD: https://github.com/rubysec/bundler-audit/blob/master/lib/bundler/audit/scanner.rb#L42

until this is resolved, I need the ability to run this check and not assume the current working directory has the .bundle and Gemfile.lock

see this run from one directory above www (where the Gemfile.lock exists)

12:27 $ pre-commit run bundler-audit --all-files --verbose
[bundler-audit] Patch-level verification for bundler.......................Failed
hookid: bundler-audit

args: --update

Updating ruby-advisory-db ...
From https://github.com/rubysec/ruby-advisory-db
 * branch            master     -> FETCH_HEAD
Already up to date.
Updated ruby-advisory-db
ruby-advisory-db: 331 advisories
/Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-audit-0.6.0/lib/bundler/audit/scanner.rb:43:in `read': No such file or directory @ rb_sysopen - /Users/dstull/rails/configbuilder/Gemfile.lock (Errno::ENOENT)
	from /Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-audit-0.6.0/lib/bundler/audit/scanner.rb:43:in `initialize'
	from /Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-audit-0.6.0/lib/bundler/audit/cli.rb:41:in `new'
	from /Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-audit-0.6.0/lib/bundler/audit/cli.rb:41:in `check'
	from /Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/thor-0.20.3/lib/thor/command.rb:27:in `run'
	from /Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/thor-0.20.3/lib/thor/invocation.rb:126:in `invoke_command'
	from /Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/thor-0.20.3/lib/thor.rb:387:in `dispatch'
	from /Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/thor-0.20.3/lib/thor/base.rb:466:in `start'
	from /Users/dstull/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-audit-0.6.0/bin/bundle-audit:10:in `<top (required)>'
	from /Users/dstull/.rbenv/versions/2.3.1/bin/bundle-audit:23:in `load'
	from /Users/dstull/.rbenv/versions/2.3.1/bin/bundle-audit:23:in `<main>'

it would be nice if the run here accepted an argument that would be the directory that those files exist in, change directory, run check, then come back to original directory after the hook has ran