added require URL

[fabiel-leon]

Avoid error in node <=8.4 ReferenceError: URL is not defined at formatEnclosure (/node_modules/feed/lib/rss2.js:153:70) at /node_modules/feed/lib/rss2.js:131:30 at Array.map () at Object.exports.default (/node_modules/feed/lib/rss2.js:83:15) at Feed.rss2 (/node_modules/feed/lib/feed.js:19:56) at module.exports (/node_modules/rss-rewriter/rss.js:92:19) at at process._tickCallback (internal/process/next_tick.js:188:7)

[coveralls]

Coverage increased (+0.002%) to 99.573% when pulling 1dc0c369995b8b70194959fba50ade41bde86e97 on fabiel-leon:master into 96c6aec2f697f8260f8fd5598b9a4edf7a2fa5e5 on jpmonette:master.

[jpmonette]

@fabiel-leon As discussed in contribution, can you make sure there is an issue describing the underlying problem?