(maint) Redirect dmidecode stderr to /dev/null

Previously most calls to dmidecode had stderr redirected to /dev/null.
dmidecode tends to have error messages that go to stderr during execution,
which pollutes facter runs. This commit updates the remaining calls to
dmidecode to also redirect stderr to /dev/null so that facter runs can continue
in blissful ignorance of any dmidecode errors.
Merge pull request #596 from haus/stable

(maint) Redirect dmidecode stderr to /dev/null
(maint) Update virtual spec to match dmidecode invocation

A previous commit updated the dmidecode invocation in util/virtual.rb, but it
did not update the corresponding spec test, which stubs the invocation to
return 'something'. This commit updates the virtual spec tests to stub the
updated invocation.
Merge pull request #601 from haus/maint/stable/fix_virtual_specs

(maint) Update virtual spec to match dmidecode invocation
(packaging) Update FACTERVERSION to 1.7.5-rc1
Merge remote-tracking branch 'origin/stable' into facter-2
Merge pull request #600 from adrienthebo/feature/facter-2/fact-65-named-resolutions

Feature/facter 2/fact 65 named resolutions
(FACT-65) Extract Resolvable and Suitable mixins

The Facter::Util::Resolution class contained behavior for determining if
a resolution was suitable, the precedence of the resolution, code for
safely resolving values, and code for actually resolving information.
This commit breaks up the Resolution class into a Suitable mixin for
determining if a resolution can run and the precedence of that
resolution, and a Resolvable mixin for handling behavior around the
resolution of a given fact.
(maint) Remove normalization tests from resolvable
(FACT-65) Add directed graph for handling dependencies
(FACT-65) Add aggregate resolution class