Fix Rubocop lints in gemspec

Prefer $LOAD_PATH over $: as it is more descriptive.

Change-Id: I5b6ce760fd5d0d8be5e9039007155fe1e04c9ed1
Reviewed-on: http://gerrit.causes.com/36170
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>