Fix service is_enabled test when using under()

[adammw]

Currently running something like:

expect(service('foo')).to be_enabled.under('runit')

throws something like:

NotImplementedError:
    check_is_enabled_runit is not implemented in Specinfra::Command::Ubuntu::V15::Service

This is because the actual method is called check_is_enabled_under_runit.

This PR fixes the method name in a way that previous hacks using expect(service('foo')).to be_enabled.under('under_runit') will still work.

[mizzy]

Good catch. Thanks!

[mizzy]

Released as v2.41.5.