Fix Rubocop lint in gemspec

Switch from using `$:` to `$LOAD_PATH`, which is much more readable.

Change-Id: Ibd13032cee0f4428c1c5cbeebff3053083423241
Reviewed-on: http://gerrit.causes.com/36106
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>