Update README.md

Unless I misread the [code] (https://github.com/puppetlabs/puppetlabs-concat/blob/master/manifests/fragment.pp#L61-L63), only 'present' or 'absent' should be used. All other options are deprecated ?
Merge pull request #214 from PierreR/patch-1

Update README.md
Lack of idempotency with `ensure => 'absent'`

Need to add an unless to the exec in the case we have `ensure =>
'absent'` for the concat, otherwise the exec (of true) runs every time.
Merge pull request #215 from mhaskel/idempotency

Lack of idempotency with `ensure => 'absent'`
Stop checking for empty stderr
Merge pull request #216 from mhaskel/test_fixes

Stop checking for empty stderr
Missed two stderr checks
Merge pull request #217 from mhaskel/more_test_failures

Missed two stderr checks