Ignore GIT_TEMPLATE_DIR environment variable in specs

Explicitly specify the template directory as an empty string in specs so
that Overcommit spec runs aren't effected by the `GIT_TEMPLATE_DIR`
environment variable.
Remove unused check-gemfile script

The `BundleCheck` post-checkout hook works without this script, so it
can be removed.
Split Whitespace check into HardTabs and TrailingWhitespace

This will make the check more specific, and give users greater control
over which checks they can disable.
Remove old plugins directory

The two hooks still in here will be ported to their respective
repository, as they are not workflow-specific.