Added simple implementation to check if git is installed before

executing commands.

spec/homesick_cli_spec.rb

@@ -13,7 +13,7 @@ describe Homesick::CLI do
 
   before { allow(homesick).to receive(:repos_dir).and_return(castles) }
 
-  describe 'smoke test' do
+  describe 'smoke tests' do
     context 'when running bin/homesick' do
       before do
         bin_path = Pathname.new(__FILE__).parent.parent
@@ -23,6 +23,17 @@ describe Homesick::CLI do
         expect(@output.length).to be > 0
       end
     end
+
+    context 'when git is not installed' do
+      before do
+        # The following line suppresses Thor warnings about overriding methods.
+        allow($stdout).to receive(:write).at_least(:once)
+        expect_any_instance_of(Homesick::CLI).to receive(:`).and_return('')
+      end
+      it 'should raise an exception when Git is not installed' do
+        Capture.stdout{ expect{Homesick::CLI.new}.to raise_error SystemExit }
+      end
+    end
   end
 
   describe 'clone' do