Upgrade test dependencies.

Gets rid of a bunch of deprecation warnings when running the test suite
under Ruby 2.4.

Gemfile.lock

@@ -3,26 +3,27 @@ GEM
   specs:
     blankslate (3.1.3)
     coderay (1.1.0)
-    diff-lcs (1.2.5)
+    diff-lcs (1.3)
     method_source (0.8.2)
     parslet (1.6.2)
       blankslate (>= 2.0, <= 4.0)
     pry (0.9.12.6)
       coderay (~> 1.0)
       method_source (~> 0.8)
       slop (~> 3.4)
-    rspec (3.1.0)
-      rspec-core (~> 3.1.0)
-      rspec-expectations (~> 3.1.0)
-      rspec-mocks (~> 3.1.0)
-    rspec-core (3.1.7)
-      rspec-support (~> 3.1.0)
-    rspec-expectations (3.1.2)
+    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 (~> 3.1.0)
-    rspec-mocks (3.1.3)
-      rspec-support (~> 3.1.0)
-    rspec-support (3.1.2)
+      rspec-support (~> 3.5.0)
+    rspec-mocks (3.5.0)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.5.0)
+    rspec-support (3.5.0)
     slop (3.4.7)
 
 PLATFORMS
@@ -34,4 +35,4 @@ DEPENDENCIES
   rspec
 
 BUNDLED WITH
-   1.11.2
+   1.13.7