Remove deprecated calls to `File.exists?`

They emit Ruby warnings on Ruby 2.1.0

lib/hub/commands.rb

@@ -998,7 +998,7 @@ help
     # included after the __END__ of the file so we can grab it using
     # DATA.
     def hub_raw_manpage
-      if File.exists? file = File.dirname(__FILE__) + '/../../man/hub.1'
+      if File.exist? file = File.dirname(__FILE__) + '/../../man/hub.1'
         File.read(file)
       else
         DATA.read
@@ -1085,7 +1085,7 @@ help
     # the pullrequest_editmsg_file, which newer hub would pick up and
     # misinterpret as a message which should be reused after a failed PR.
     def valid_editmsg_file?(message_file)
-      File.exists?(message_file) &&
+      File.exist?(message_file) &&
         File.mtime(message_file) > File.mtime(__FILE__)
     end
 

test/standalone_test.rb

@@ -7,15 +7,15 @@ class StandaloneTest < Minitest::Test
   include FileUtils
 
   def setup
-    rm "hub" if File.exists? 'hub'
-    rm_rf "/tmp/_hub_private" if File.exists? '/tmp/_hub_private'
+    rm "hub" if File.exist? 'hub'
+    rm_rf "/tmp/_hub_private" if File.exist? '/tmp/_hub_private'
     mkdir "/tmp/_hub_private"
     chmod 0400, "/tmp/_hub_private"
   end
 
   def teardown
-    rm "hub" if File.exists? 'hub'
-    rm_rf "/tmp/_hub_private" if File.exists? "/tmp/_hub_private"
+    rm "hub" if File.exist? 'hub'
+    rm_rf "/tmp/_hub_private" if File.exist? "/tmp/_hub_private"
   end
 
   def test_standalone