feat(aws-lambda): Add python 3.7 runtime (#1379)

This runtime was announced on Nov 19 and had been missed from the
Runtime list in the CDK, this adds it in. Tested inline editing as
working in the console so left that enabled with this runtime also.

packages/@aws-cdk/aws-lambda/lib/runtime.ts

@@ -28,6 +28,7 @@ export class Runtime {
   public static readonly NodeJS810 =    new Runtime('nodejs8.10',     RuntimeFamily.NodeJS, { supportsInlineCode: true });
   public static readonly Python27 =     new Runtime('python2.7',      RuntimeFamily.Python, { supportsInlineCode: true });
   public static readonly Python36 =     new Runtime('python3.6',      RuntimeFamily.Python, { supportsInlineCode: true });
+  public static readonly Python37 =     new Runtime('python3.7',      RuntimeFamily.Python, { supportsInlineCode: true });
   public static readonly Java8 =        new Runtime('java8',          RuntimeFamily.Java);
   public static readonly DotNetCore1 =  new Runtime('dotnetcore1.0',  RuntimeFamily.DotNetCore);
   public static readonly DotNetCore2 =  new Runtime('dotnetcore2.0',  RuntimeFamily.DotNetCore);