(maint) Explicitly set LANG in RPM spec file

LANG is set to 'C' by default, which is problematic because numerous
parts of Ruby have to parse UTF8 files. This will solve a problem with
Fedora 19, which has an umlaut in the release name. If LANG is
set to 'C', the umlaut then leads to unexpected values returned from Facter.
Merge pull request #884 from mckern/1.6.x

(maint) Explicitly set LANG in RPM spec file