Bump sentry-raven gem to 2.7.4

Contains only minor fixes from 2.7.2:
https://github.com/getsentry/raven-ruby/releases

Gemfile.lock

@@ -807,7 +807,7 @@ GEM
     selenium-webdriver (3.12.0)
       childprocess (~> 0.5)
       rubyzip (~> 1.2)
-    sentry-raven (2.7.2)
+    sentry-raven (2.7.4)
       faraday (>= 0.7.6, < 1.0)
     settingslogic (2.0.9)
     sexp_processor (4.11.0)