[Config] Use the pwd as installation root if no Podfile can be found

lib/cocoapods/config.rb

@@ -134,6 +134,7 @@ module Pod
             current_path = current_path.parent
           end
         end
+        @installation_root ||= Pathname.pwd
       end
       @installation_root
     end
@@ -181,7 +182,7 @@ module Pod
     # @return [Nil]
     #
     def podfile_path
-      @podfile_path ||= podfile_path_in_dir(installation_root) if installation_root
+      @podfile_path ||= podfile_path_in_dir(installation_root)
     end
 
     # Returns the path of the Lockfile.

spec/unit/config_spec.rb

@@ -59,9 +59,9 @@ module Pod
         end
       end
 
-      it "it returns a nil installation root if no Podfile can be found" do
+      it "it returns the working directory as the installation root if no Podfile can be found" do
         Dir.chdir(temporary_directory) do
-          config.installation_root.should == nil
+          config.installation_root.should == temporary_directory
         end
       end