Adress @jmarantz his comment from #1453's review (#1458)

PR #1453 was merged too soon, using this PR to address
https://github.com/pagespeed/ngx_pagespeed/pull/1453#discussion_r134222719

(Sorry Josh!)

src/ngx_pagespeed.cc

@@ -90,6 +90,7 @@ net_instaweb::NgxRewriteDriverFactory* active_driver_factory = NULL;
 namespace net_instaweb {
 
 const char* kInternalEtagName = "@psol-etag";
+const int kNginx1_13_4 = 1013004;
 // The process context takes care of proactively initialising
 // a few libraries for us, some of which are not thread-safe
 // when they are initialized lazily.
@@ -3022,7 +3023,7 @@ ngx_int_t ps_preaccess_handler(ngx_http_request_t* r) {
 
   // move handlers before try_files && content phase
   // As of nginx 1.13.4 we will be right before the try_files module
-  #if (nginx_version < 1013004)
+  #if (nginx_version < kNginx1_13_4)
   while (ph[i + 1].checker != ngx_http_core_try_files_phase &&
          ph[i + 1].checker != ngx_http_core_content_phase) {
     ph[i] = ph[i + 1];
@@ -3088,7 +3089,7 @@ ngx_int_t ps_init(ngx_conf_t* cf) {
     int phase = NGX_HTTP_PRECONTENT_PHASE;
 
     // As of nginx 1.13.4, try_files has changed.
-#if (nginx_version < 1013004)
+#if (nginx_version < kNginx1_13_4)
     phase = NGX_HTTP_PREACCESS_PHASE;
 #endif