Test against the newset version of ruby 2.1 (2.1.8)
Test against the newset version of ruby 2.1 (2.1.8)

Travis CI runs against ruby 2.1.5 when writing `rvm: 2.1`
and I think which is not we indend to test against.
I know it is boring work to update the file along with
every 2.1.x patch release, but it is necessary to keep
test environments to be latest :(

amatsuda/active_decorator,yui-knk/active_decorator,yui-knk/active_decorator,amatsuda/active_decorator