Fix scalable-react#75: Update template for stateless components

The `{{else if ...}}` construct is not supported prior to handlebars v3.0.0. Based on yarn.lock, seems like plop is
using handlebars 2.0.0.

* Replace `{{else if ...}}` with a nested `if/else` block
* Remove unnecessary branch point on `{{#unless wantFlowTypes}}` which will never be reached since that condition is
  already checked in an earlier `if` condition.

config/generators/component/stateless.js.hbs

@@ -26,16 +26,15 @@ function {{ properCase name }}(props: {}) {
     </Box>
   );
 }
-{{else if wantPropTypes}}
-{{#unless wantFlowTypes}}
+{{else}}
+{{#if wantPropTypes}}
 function {{ properCase name }}(props) {
   return (
     <Box>
 
     </Box>
   );
 }
-{{/unless}}
 {{else}}
 function {{ properCase name }}() {
   return (
@@ -45,6 +44,7 @@ function {{ properCase name }}() {
   );
 }
 {{/if}}
+{{/if}}
 
 {{#if wantPropTypes}}
 {{ properCase name }}.propTypes = {