Update dependency better_errors to v2.8.0 [SECURITY]

Gemfile.lock

@@ -7,7 +7,7 @@ GEM
       minitest (~> 5.1)
       tzinfo (~> 1.1)
     backports (3.15.0)
-    better_errors (2.5.1)
+    better_errors (2.9.1)
       coderay (>= 1.0.0)
       erubi (>= 1.0.0)
       rack (>= 0.9.0)
@@ -20,7 +20,7 @@ GEM
     coffee-script-source (1.12.2)
     concurrent-ruby (1.1.9)
     daemons (1.4.0)
-    erubi (1.9.0)
+    erubi (1.10.0)
     ethon (0.12.0)
       ffi (>= 1.3.0)
     eventmachine (1.2.7)