Add an is_admin() check to our site request cache clear

This causes some issues if we're using WP-CLI. In the future we
can get a bit smarter about how this is handled, so that if a
site is created or modified through WP-CLI, we still clear this
cache
Merge pull request #214 from washingtonstateuniversity/cli-import

[PR] Add an is_admin() check to our site request cache clear