Fix Version.IsValid.

Fixes #15.

version.go

@@ -42,7 +42,7 @@ func (v Version) Contains(other Version) bool {
 }
 
 func (v Version) IsValid() bool {
-	return v == InvalidVersion
+	return v != InvalidVersion
 }
 
 var InvalidVersion = Version{-1, -1, -1}

version_test.go

@@ -106,3 +106,8 @@ func (s *VersionSuite) TestContains(c *C) {
 		}
 	}
 }
+
+func (s *VersionSuite) TestIsValid(c *C) {
+	c.Assert(InvalidVersion.IsValid(), Equals, false)
+	c.Assert(Version{0, 0, 0}.IsValid(), Equals, true)
+}