spec: Replace deprecated File.exists? with File.exist? (#550)

spec/lib/mini_magick/image_spec.rb

@@ -124,7 +124,7 @@ def create(path = image_path)
 
         it "creates an image" do
           image = create
-          expect(File.exists?(image.path)).to eq true
+          expect(File.exist?(image.path)).to eq true
         end
 
         it "validates the image if validation is set" do
@@ -263,12 +263,12 @@ def create(path = image_path)
           cache_path = image.path.sub(/mpc$/, "cache")
           image.format("png")
 
-          expect(File.exists?(cache_path)).to eq false
+          expect(File.exist?(cache_path)).to eq false
         end
 
         it "doesn't delete itself when formatted to the same format" do
           subject.format(subject.type.downcase)
-          expect(File.exists?(subject.path)).to eq true
+          expect(File.exist?(subject.path)).to eq true
         end
 
         it "reformats multi-image formats to multiple images" do
@@ -731,22 +731,22 @@ def create(path = image_path)
           image = described_class.open(image_path)
           image.destroy!
 
-          expect(File.exists?(image.path)).to eq false
+          expect(File.exist?(image.path)).to eq false
         end
 
         it "doesn't delete when there is no tempfile" do
           image = described_class.new(image_path)
           image.destroy!
 
-          expect(File.exists?(image.path)).to eq true
+          expect(File.exist?(image.path)).to eq true
         end
 
         it "deletes .cache files generated by handling .mpc files" do
           image = described_class.open(image_path)
           image.format("mpc")
           image.destroy!
 
-          expect(File.exists?(image.path.sub(/mpc$/, "cache"))).to eq false
+          expect(File.exist?(image.path.sub(/mpc$/, "cache"))).to eq false
         end
       end