don't use acme-client from the repo

Gemfile.lock

@@ -8,16 +8,11 @@ GIT
       json (~> 1.7)
       rack (>= 1.4)
 
-GIT
-  remote: https://github.com/unixcharles/acme-client
-  revision: 9a80e9301acd3f17ebff6ceaf245e0c400d08519
-  specs:
-    acme-client (0.5.0)
-      faraday (~> 0.9, >= 0.9.1)
-
 GEM
   remote: https://rubygems.org/
   specs:
+    acme-client (0.5.5)
+      faraday (~> 0.9, >= 0.9.1)
     actioncable (5.0.2)
       actionpack (= 5.0.2)
       nio4r (>= 1.2, < 3.0)
@@ -232,7 +227,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
-  acme-client!
+  acme-client
   annotate
   authie (~> 3.0)
   autoprefixer-rails