Do not raise error when Gemfile does not exist

This is to allow user to run `appraisal help` to get the help message.

Note that there is no test for this, as it's impossible to trick Bundler
to run the executable without a Gemfile. I tested it locally and it
works, so that has to be enough.

Fix #98

lib/appraisal/gemfile.rb

@@ -10,7 +10,9 @@ module Appraisal
   # Load bundler Gemfiles and merge dependencies
   class Gemfile < BundlerDSL
     def load(path)
-      run(IO.read(path))
+      if ::File.exist?(path)
+        run(IO.read(path))
+      end
     end
 
     def run(definitions)

spec/appraisal/file_spec.rb

@@ -7,6 +7,7 @@
 
 describe Appraisal::File do
   it "complains when no Appraisals file is found" do
+    allow(::File).to receive(:exist?).with(/Gemfile/).and_return(true)
     allow(::File).to receive(:exist?).with("Appraisals").and_return(false)
     expect { described_class.new }.to raise_error(Appraisal::AppraisalsNotFound)
   end