Fix RSpec tests so that they run successfully under docker containers

[juniorsysadmin]

The RSpec tests can't currently be run on TravisCI with sudo: false as sadly one of the provider tests fail when this is set. Given how slow tests now run on non-docker TravisCI instances, fixing this would without doubt result in an immediate 50% increase in general health and well-being for any PR contributor.

[igalic]

\o/

[juniorsysadmin]

For those playing at home, the culprit when invoked on TravisCI is:
`
Failures:

1. Puppet::Type::Package::ProviderNpm when npm packages are installed globally should log and return no packages if JSON isn't output
   Failure/Error: expect(@provider.class.instances).to eq([])
   Mocha::ExpectationError:
   unexpected invocation: Puppet.debug('Warning: npm list --json exited with code 126')
   unsatisfied expectations:
   • expected exactly once, not yet invoked: Puppet.debug(regexp_matches(/npm list.*failure!/))
     `

[juniorsysadmin]

A hack fix has been done in #156