Fix build on clean environment (#425)

Dependency on BuildNumber is not documented so we should set up some
working defaults. Build on environment where BuildNumber variable
is defined should work as before.

Jint/Directory.Build.props

@@ -4,6 +4,7 @@
     <PackageId>Jint</PackageId>
     <AssemblyTitle>Jint</AssemblyTitle>
     <Description>Javascript interpreter for .NET which provides full ECMA 5.1 compliance.</Description>
+    <BuildNumber Condition="'$(BuildNumber)' == ''">0</BuildNumber>
     <VersionPrefix>2.11.$(BuildNumber)</VersionPrefix>
     <VersionSuffix></VersionSuffix>
     <PackageId>Jint</PackageId>