add version to --show-config so it is easily parseable

lib/config.js

@@ -30,7 +30,8 @@ async function getAppiumConfig () {
   let built = stat.mtime.getTime();
   let config = {
     'git-sha': await getGitRev(),
-    'built': built
+    'built': built,
+    'version': APPIUM_VER,
   };
   return config;
 }

test/config-specs.js

@@ -32,6 +32,7 @@ describe('Config', () => {
         config.should.be.an('object');
         should.exist(config['git-sha']);
         should.exist(config.built);
+        should.exist(config.version);
       });
     });
     describe('showConfig', () => {