Upgrade rspec to latest 3.x version & fix deprecation warnings

Closes #2

Author: sohaibbhatti

method_log.gemspec

@@ -26,5 +26,5 @@ Gem::Specification.new do |s|
   s.add_dependency 'trollop'
 
   s.add_development_dependency 'rake'
-  s.add_development_dependency 'rspec', '~> 2.0'
+  s.add_development_dependency 'rspec', '~> 3.0'
 end

spec/method_diff_spec.rb

@@ -9,8 +9,8 @@ module MethodLog
     let(:diff) { MethodDiff.new(first_commit, second_commit) }
 
     it 'generates text diff of the method source for two commits' do
-      first_commit.stub(:method_source).and_return(%{line 1\nline 2\n})
-      second_commit.stub(:method_source).and_return(%{line 2\nline 3\n})
+      allow(first_commit).to receive(:method_source).and_return(%{line 1\nline 2\n})
+      allow(second_commit).to receive(:method_source).and_return(%{line 2\nline 3\n})
       expect(diff.to_s).to eq(%{-line 1\n line 2\n+line 3\n})
     end
   end

spec/source_file_spec.rb

@@ -39,7 +39,7 @@ def bar
     end
 
     it 'looks up source in repository using SHA if no source set' do
-      repository.stub(:lookup).with(sha).and_return(blob)
+      allow(repository).to receive(:lookup).with(sha).and_return(blob)
       file = source(path: 'path/to/source.rb', repository: repository, sha: sha)
       expect(file.source).to eq('source')
     end