Pin sinatra gem

Sinatra 2.0.0 was released on the 7th of May 2017.  It requires rack ~>
2.0 and *that* requires ruby 2.2.  Puppet 4 AIO ships with ruby 2.1, so
pin sinatra to prevent errors like.

```
Error: Execution of '/opt/puppetlabs/puppet/bin/gem install --no-rdoc
--no-ri sinatra' returned 1: ERROR:  Error installing sinatra:
        rack requires Ruby version >= 2.2.2.
```

Fixes voxpupuli#140

manifests/params.pp

@@ -148,6 +148,7 @@
   $webhook_configfile_mode       = '0644'
   $webhook_ignore_environments   = []
   $webhook_mco_arguments         = undef
+  $webhook_sinatra_version       = '~> 1.0'      # Sinatra 2 requires rack 2 which in turn requires ruby 2.2. Puppet 4 AIO ships with ruby 2.1
 
   # Service Settings for SystemD in EL7
   if $::osfamily == 'RedHat' and $::operatingsystemmajrelease == '7' {

manifests/webhook/package.pp

@@ -1,12 +1,13 @@
 # Private class, do not include it directly.
 # Installs the webhook packages
 class r10k::webhook::package (
-  $is_pe_server = $r10k::params::is_pe_server,
-  $provider     = $r10k::params::provider,
+  $is_pe_server    = $r10k::params::is_pe_server,
+  $provider        = $r10k::params::provider,
+  $sinatra_version = $r10k::params::webhook_sinatra_version,
 ) inherits r10k::params {
   if !defined(Package['sinatra']) {
     package { 'sinatra':
-      ensure   => installed,
+      ensure   => $sinatra_version,
       provider => $provider,
       before   => Service['webhook'],
     }