Apply Layout/LineLength rubocop rule

Note we're excluding a few files:

* Configuration files that weren't generated by us
* Migration files that weren't generated by us
* The Gemfile, since it includes an important comment that must be on
  the same line as the gem declaration
* The Budget::Stats class, since the heading statistics are a mess and
  having shorter lines would require a lot of refactoring

spec/i18n_spec.rb

@@ -7,13 +7,13 @@ describe "I18n" do
   let(:unused_keys) { i18n.unused_keys }
 
   it "does not have missing keys" do
-    expect(missing_keys).to be_empty,
-                            "Missing #{missing_keys.leaves.count} i18n keys, run `i18n-tasks missing' to show them"
+    expect(missing_keys).to be_empty, "Missing #{missing_keys.leaves.count} i18n keys, " \
+                                      "run `i18n-tasks missing' to show them"
   end
 
   it "does not have unused keys" do
-    expect(unused_keys).to be_empty,
-                           "#{unused_keys.leaves.count} unused i18n keys, run `i18n-tasks unused' to show them"
+    expect(unused_keys).to be_empty, "#{unused_keys.leaves.count} unused i18n keys, " \
+                                     "run `i18n-tasks unused' to show them"
   end
 
   context "Plurals" do