[[FIX]] Preserve functionality in "legacy" Node.js

A recent minor release of the npm module `request` has introduced
breaking changes for Node.js versions 0.10 and 0.12. Because JSHint
depends on this module transitively through one of its
`devDependencies`, this interferes with the project's ability to verify
its own correctness in the effected environments.

Allow JSHint to be installed in legacy environments by specifying the
effected dependencies as "optional." These dependencies are only
required by tests for the PhantomJS platform, and those tests can be
orchestrated from a single actively-maintained version of Node.js
without effecting coverage.

package.json

@@ -66,13 +66,16 @@
     "jscs":                           "1.11.x",
     "mock-stdin":                     "0.3.x",
     "nodeunit":                       "0.9.x",
-    "phantom":                        "~4.0.1",
-    "phantomjs-prebuilt":             "~2.1.7",
     "regenerate":                     "1.2.x",
     "sinon":                          "1.12.x",
     "unicode-6.3.0":                  "0.1.x"
   },
 
+  "optionalDependencies": {
+    "phantom":                        "~4.0.1",
+    "phantomjs-prebuilt":             "~2.1.7"
+  },
+
   "license": "(MIT AND JSON)",
 
   "preferGlobal": true,

tests/browser.js

@@ -1,13 +1,24 @@
 "use strict";
 
-var phantom = require("phantom");
+var phantom, phantomJsPrebuilt;
+try {
+  phantom = require("phantom");
+  phantomJsPrebuilt = require("phantomjs-prebuilt");
+} catch (err) {
+  throw new Error(
+    "Unable to run tests in PhantomJS because the required dependencies are " +
+    "not available. Please note that JSHint does not support development " +
+    "using versions of Node.js which are no longer maintained."
+  );
+}
+
 var createTestServer = require("./helpers/browser/server");
 var options = {
   /**
    * The `phantom` module provides a Node.js API for the PhantomJS binary,
    * while the `phantomjs` module includes the binary itself.
    */
-  phantomPath: require("phantomjs-prebuilt").path
+  phantomPath: phantomJsPrebuilt.path
 };
 var port = process.env.NODE_PORT || 8045;
 var ph;