Update ruby from 2.3.1 to 2.4.0 (citation-style-language#2484)

.travis.yml

@@ -2,7 +2,7 @@ sudo: false
 language: ruby
 cache: bundler
 rvm:
-  - 2.3.1
+  - 2.4.0
 notifications:
   email:
     recipients:

Gemfile.lock

@@ -3,31 +3,31 @@ GEM
   specs:
     csl (1.4.5)
       namae (~> 0.7)
-    diff-lcs (1.2.5)
-    fuubar (2.0.0)
-      rspec (~> 3.0)
+    diff-lcs (1.3)
+    fuubar (2.2.0)
+      rspec-core (~> 3.0)
       ruby-progressbar (~> 1.4)
-    mini_portile2 (2.0.0)
-    namae (0.11.0)
-    nokogiri (1.6.7.2)
-      mini_portile2 (~> 2.0.0.rc2)
-    nokogiri (1.6.7.2-x86-mingw32)
-      mini_portile2 (~> 2.0.0.rc2)
-    rake (10.5.0)
-    rspec (3.4.0)
-      rspec-core (~> 3.4.0)
-      rspec-expectations (~> 3.4.0)
-      rspec-mocks (~> 3.4.0)
-    rspec-core (3.4.3)
-      rspec-support (~> 3.4.0)
-    rspec-expectations (3.4.0)
+    mini_portile2 (2.1.0)
+    namae (0.11.3)
+    nokogiri (1.7.0.1)
+      mini_portile2 (~> 2.1.0)
+    nokogiri (1.7.0.1-x86-mingw32)
+      mini_portile2 (~> 2.1.0)
+    rake (12.0.0)
+    rspec (3.5.0)
+      rspec-core (~> 3.5.0)
+      rspec-expectations (~> 3.5.0)
+      rspec-mocks (~> 3.5.0)
+    rspec-core (3.5.4)
+      rspec-support (~> 3.5.0)
+    rspec-expectations (3.5.0)
       diff-lcs (>= 1.2.0, < 2.0)
-      rspec-support (~> 3.4.0)
-    rspec-mocks (3.4.1)
+      rspec-support (~> 3.5.0)
+    rspec-mocks (3.5.0)
       diff-lcs (>= 1.2.0, < 2.0)
-      rspec-support (~> 3.4.0)
-    rspec-support (3.4.1)
-    ruby-progressbar (1.7.5)
+      rspec-support (~> 3.5.0)
+    rspec-support (3.5.0)
+    ruby-progressbar (1.8.1)
 
 PLATFORMS
   ruby