Add Uglify as devDependencies

If you don't have `uglify-js` installed globally, you get a bad error message. Its probably better to just install `uglify-js` along with the development dependencies and always use that when you run `make`. That way everything is self contained.

```
$ make
cat lib/banner.js lib/polyfill-wrapper-start.js > dist/system.js;
for extension in meta register core global cjs amd map plugins bundles versions; do \
		cat lib/extension-$extension.js >> dist/system.js; \
	done
for extension in meta register core global cjs amd map plugins bundles versions; do \
		echo $extension"(System);" >> dist/system.js; \
	done
cat lib/polyfill-wrapper-end.js >> dist/system.js;
cat lib/banner.js lib/polyfill-wrapper-start.js > dist/system-production-csp.js;
for extension in scriptLoader register core cjs amd map bundles versions; do \
		cat lib/extension-$extension.js >> dist/system-production-csp.js; \
	done
for extension in scriptLoader register core cjs amd map bundles versions; do \
		echo $extension"(System);" >> dist/system-production-csp.js; \
	done
cat lib/polyfill-wrapper-end.js >> dist/system-production-csp.js;
cat lib/banner.js > dist/system.min.js;
uglifyjs dist/system.js -cm >> dist/system.min.js
nodenv: `uglifyjs' does not exist for v0.10
make: *** [uglify] Error 1
```

/cc @guybedford