Fix #318: Introducing custom uninstall in sensu_gem

sensu_gem inherits the methods from its parent, gem.rb.
The uninstall method in gem.rb contains a bug that prevents the right
gem command from being issued. (/usr/bin/gem uninstall -x -a
sensu-plugin is executed instead of /opt/sensu/embedded/bin/gem
uninstall -x -a sensu-plugin).

Issue PUP-4191 has been raised in the puppet project and PR #3708 has
been opened in puppetlabs/puppet.

As temporary workaround, we should overwrite the gem.rb uninstall
method to use the execute function, which then issues the right gem
command.

lib/puppet/provider/package/sensu_gem.rb

@@ -12,4 +12,14 @@
   has_feature :versionable, :install_options
 
   commands :gemcmd => "/opt/sensu/embedded/bin/gem"
+
+  def uninstall
+    command = [command(:gemcmd), "uninstall"]
+    command << "-x" << "-a" << resource[:name]
+    output = execute(command)
+
+    # Apparently some stupid gem versions don't exit non-0 on failure
+    self.fail "Could not uninstall: #{output.chomp}" if output.include?("ERROR")
+  end
+
 end