Raise an appropriate error when a gem is specified using a symbol.

Previously, bundler would raise an error like "Could not find gem 'foo' in any of the gem sources", which was misleading and confusing.  This closes #434.

lib/bundler/dsl.rb

@@ -44,6 +44,10 @@ def gemspec(opts)
     end
 
     def gem(name, *args)
+      if name.is_a?(Symbol)
+        raise GemfileError, %{You need to specify gem names as Strings. Use 'gem "#{name.to_s}"' instead.}
+      end
+
       options = Hash === args.last ? args.pop : {}
       version = args.last || ">= 0"
       if group = options[:groups] || options[:group]

spec/install/gems/simple_case_spec.rb

@@ -21,6 +21,14 @@
       should_be_installed("rack 1.0.0")
     end
 
+    it "raises an appropriate error when gems are specified using symbols" do
+      status = install_gemfile(<<-G, :exit_status => true)
+        source "file://#{gem_repo1}"
+        gem :rack
+      G
+      status.should == 4
+    end
+
     it "pulls in dependencies" do
       install_gemfile <<-G
         source "file://#{gem_repo1}"