Fix flake8 staged file paths

Similar to d55a040ba, this is a fallout from changing the signature of
`staged` back to `StagedFile` instances.

I also standardized on the `collect(:path)` syntax to make things more
consistent.

Change-Id: Ic2210fc2508c2bbd277aed8f14521a5b6c428e5f
Reviewed-on: https://gerrit.causes.com/27375
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>
Remove erb_syntax pre-commit hook

It doesn't work as intended, and all the cool kids are using Haml.

Fixes: https://github.com/causes/overcommit/issues/13

Change-Id: I4bf355ae267b78042e71b780d8edb966b48c78bb
Reviewed-on: https://gerrit.causes.com/27376
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>