Set DLR as specific version (#327)

Author: slide

Package/nuget/IronPython.nuspec

@@ -15,7 +15,7 @@
     <tags>ironpython python dynamic dlr</tags>
     <dependencies>
       <group>
-        <dependency id="DynamicLanguageRuntime" version="1.2.1" />
+        <dependency id="DynamicLanguageRuntime" version="[1.2.1]" />
       </group>
       <group targetFramework="netcoreapp2.0">
         <dependency id="System.Text.Encoding.CodePages" version="4.4.0" />