Add flatDir dependency unit test

The test fails with my fix for #36, which passed the provided example
project, due to an NPE using the comparator. The test passes with the
fix provided by #59, validating @dbachelder's solution.

README.md

@@ -1,6 +1,6 @@
 # Gradle Versions Plugin
 
-In the spirit of the [Maven Versions Plugin](http://mojo.codehaus.org/versions-maven-plugin/), 
+In the spirit of the [Maven Versions Plugin](http://mojo.codehaus.org/versions-maven-plugin/),
 this plugin provides a task to determine which dependencies have updates.
 
 ## Usage
@@ -15,14 +15,14 @@ buildscript {
   repositories {
     jcenter()
   }
-  
+
   dependencies {
     classpath 'com.github.ben-manes:gradle-versions-plugin:0.9'
     // classpath 'org.codehaus.groovy:groovy-backports-compat23:2.3.5' // uncomment if you're using Gradle 1.x
   }
 }
 ```
-The current version is known to work with Gradle versions up to 2.4-rc-1.
+The current version is known to work with Gradle versions up to 2.4.
 
 ## Tasks
 

gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.3-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-2.4-bin.zip

src/test/groovy/com/github/benmanes/gradle/versions/DependencyUpdatesSpec.groovy

@@ -266,6 +266,33 @@ class DependencyUpdatesSpec extends Specification {
 	    unresolved == 2
 	}
 
+  def 'Single project with flatDir repository'() {
+    given:
+      def project = singleProject()
+      project.repositories {
+        flatDir {
+          dirs getClass().getResource('/libs/')
+        }
+      }
+      project.configurations {
+        flat
+      }
+      project.dependencies {
+        flat(name: 'guice-4.0', ext: 'jar')
+        flat(name: 'guava-18.0', ext: 'jar')
+      }
+    when:
+      def reporter = evaluate(project)
+      reporter.write()
+    then:
+      with(reporter) {
+        unresolved.size() == 1
+        upgradeVersions.isEmpty()
+        upToDateVersions.size() == 1
+        downgradeVersions.isEmpty()
+      }
+  }
+
   def singleProject() {
     new ProjectBuilder().withName('single').build()
   }