LG-651 Allow newrelic_rpm to be updated

[monfresh]

Why: The newrelic_rpm gem added a call to SecureRandom.hex(8)
on 6/13/18:
newrelic/newrelic-ruby-agent@60ef61c

Some of our tests stub calls to SecureRandom, and because we have
New Relic tracers in an initializer, the newly-added call to
SecureRandom in the gem caused the tests to fail because they are
not expecting that additional call.

RSpec tells us to stub the call with a default value first in this case.

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