(FACT-409) Preserve semantics of .exec

Raising an error when .exec tries to run a non-existent command is a
major backwards incompatible change. While we're on a breaking change
boundary we're trying to minimize the upgrade pain, so to reduce this
the behavior of .exec is preserved when running non-existent commands.
In the long run .exec should be deprecated in favor of .execute.
Merge pull request #654 from adrienthebo/issue/stable/fact-409-exec_compatibility

(FACT-409) Preserve semantics of .exec
(maint) Pin rake to the last 1.8 compatible version
Merge pull request #655 from adrienthebo/maint/stable/pin-rake-version

(maint) Pin rake to the last 1.8 compatible version
(maint) Update solaris zone facts to use .execute
Merge branch 'maint/stable/solaris-zone-specs' into stable
(maint) Remove extra argument from domain .exec calls
Merge branch 'maint/stable/domain-exec-args' into stable
(maint) Update fedora test version to 19
Merge branch 'defect/stable/fedora-19-acceptance' into stable
Merge remote-tracking branch 'origin/stable' into facter-2