Add sentry for error tracing

Gemfile.lock

@@ -151,7 +151,7 @@ GEM
     factory_bot_rails (5.1.1)
       factory_bot (~> 5.1.0)
       railties (>= 4.2.0)
-    faraday (1.0.0)
+    faraday (0.17.3)
       multipart-post (>= 1.2, < 3)
     faye-websocket (0.10.9)
       eventmachine (>= 0.12.0)
@@ -368,6 +368,8 @@ GEM
     selenium-webdriver (3.142.7)
       childprocess (>= 0.5, < 4.0)
       rubyzip (>= 1.2.2)
+    sentry-raven (2.13.0)
+      faraday (>= 0.7.6, < 1.0)
     shoulda-matchers (4.2.0)
       activesupport (>= 4.2.0)
     simplecov (0.17.1)
@@ -499,6 +501,7 @@ DEPENDENCIES
   rubyzip
   sass-rails
   selenium-webdriver
+  sentry-raven
   shoulda-matchers
   simplecov
   slim-rails