Update gems and yarn packages.

Remove *.scss.erb loader because it's now supported natively by
webpacker.

Gemfile.lock

@@ -149,8 +149,8 @@ GEM
     faraday (0.17.0)
       multipart-post (>= 1.2, < 3)
     ffaker (2.13.0)
-    ffi (1.11.2)
-    ffi (1.11.2-x64-mingw32)
+    ffi (1.11.3)
+    ffi (1.11.3-x64-mingw32)
     flamegraph (0.9.5)
     get_process_mem (0.2.5)
       ffi (~> 1.0)
@@ -183,7 +183,7 @@ GEM
     listen (3.2.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
-    loofah (2.3.1)
+    loofah (2.4.0)
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
     mail (2.7.1)
@@ -241,7 +241,7 @@ GEM
       mini_portile2 (~> 2.4.0)
     nokogiri (1.10.5-x64-mingw32)
       mini_portile2 (~> 2.4.0)
-    nokogumbo (2.0.1)
+    nokogumbo (2.0.2)
       nokogiri (~> 1.8, >= 1.8.4)
     ntlm-http (0.1.1)
     oauth2 (1.4.2)
@@ -397,7 +397,7 @@ GEM
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff (>= 0.4.0, < 2.0.0)
-    webpacker (4.0.7)
+    webpacker (4.2.0)
       activesupport (>= 4.2)
       rack-proxy (>= 0.6.1)
       railties (>= 4.2)