(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 tag '1.7.5-rc1'

1.7.5-rc1

* tag '1.7.5-rc1':
  (packaging) Update FACTERVERSION to 1.7.5-rc1
  (maint) Update virtual spec to match dmidecode invocation
  (maint) Redirect dmidecode stderr to /dev/null