Replace ARGV with an empty array before each spec run.
Replace ARGV with an empty array before each spec run.

Some of the specs were leaving ARGV in a non-pristine state (such as
those in runner_spec.rb), which was then implicitly used in
Thor::Base.start et al. It happened to be the case that the order in
which rspec was loading and running the specs made this not a problem,
but if the specs were run in a randomized order it could be a problem.

Closes #171.

fancyremarker/thor,sideci-sample/sideci-sample-thor,fancyremarker/thor,b-dean/thor,segiddins/thor,flyinbutrs/thor,groovenauts/thor,segiddins/thor,groovenauts/thor,linkodehub/thor,sideci-sample/sideci-sample-thor,shaiguitar/thor,doudou/thor,flyinbutrs/thor,fhernandez173/thor,fhernandez173/thor,erikhuda/thor,b-dean/thor,linkodehub/thor,r7kamura/thor,r7kamura/thor