Monkey-patch nock, add Node v8 to test matrix

.travis.yml

@@ -4,6 +4,7 @@ node_js:
   - 4
   - 6
   - 7
+  - 8
 script:
   - npm run lint
   - npm run test

test/setup.js

@@ -1,4 +1,26 @@
 var nock = require('nock');
+if (parseInt(process.versions.node, 10) >= 8) {
+  // See DEP0066 at https://nodejs.org/api/deprecations.html.
+  // _headers and _headerNames have been removed from Node v8, which causes
+  // nock <= 9.0.13 to fail. The snippet below monkey-patches the library, see
+  // https://github.com/node-nock/nock/pull/929/commits/f6369d0edd2a172024124f
+  // for the equivalent logic without proxies.
+  Object.defineProperty(require('http').ClientRequest.prototype, '_headers', {
+    get: function() {
+      var request = this;
+      // eslint-disable-next-line no-undef
+      return new Proxy(request.getHeaders(), {
+        set: function(target, property, value) {
+          request.setHeader(property, value);
+          return true;
+        },
+      });
+    },
+    set: function() {
+      // Ignore.
+    },
+  });
+}
 
 nock.enableNetConnect('127.0.0.1');