replace epoll and threaded servers with single nio4r evented server

Gemfile.lock

@@ -92,7 +92,6 @@ GEM
     deep_merge (1.1.1)
     dynamic_form (1.1.4)
     encrypto_signo (1.0.0)
-    epoll (0.3.0)
     erubis (2.7.0)
     execjs (2.7.0)
     faraday (0.9.2)
@@ -226,7 +225,6 @@ DEPENDENCIES
   createsend (~> 4.0)
   dynamic_form
   encrypto_signo
-  epoll
   foreman
   gelf
   haml
@@ -241,6 +239,7 @@ DEPENDENCIES
   mysql2 (>= 0.3.18, < 0.5)
   nifty-utils
   nilify_blanks
+  nio4r
   puma (~> 3.0)
   rails (= 5.0.2)
   sass-rails (~> 5.0)
@@ -251,4 +250,4 @@ DEPENDENCIES
   web-console
 
 BUNDLED WITH
-   1.14.5
+   1.14.6