(MODULES-1190) respect puppet-lint ignore paths in Rakefile

Without this patch applied, the PuppetLint.configuration.ignore_paths
specified in the Rakefile is discarded in favour of the hardcoded value
provided by puppetlabs_spec_helper from v0.5.0.  This patch fixes the issue
by adding the hardcoded value to the array rather than overwriting it.
Merge pull request #69 from mmickan/fix/master/MODULES-1190

(MODULES-1190) respect puppet-lint ignore paths in Rakefile