Revert removal on npmconf

This reverts commit 5e21f47.

The solution implemented was not equivalent and resulted in issues
for users with proxies.

Fixes sass#1458

package.json

@@ -64,6 +64,7 @@
     "mkdirp": "^0.5.1",
     "nan": "^2.2.0",
     "node-gyp": "^3.3.1",
+    "npmconf": "^2.1.2",
     "sass-graph": "^2.1.1"
   },
   "devDependencies": {

scripts/install.js

@@ -5,6 +5,7 @@
 var fs = require('fs'),
     eol = require('os').EOL,
     mkdir = require('mkdirp'),
+    npmconf = require('npmconf'),
     path = require('path'),
     got = require('got'),
     pkg = require('../package.json'),
@@ -61,17 +62,25 @@ function download(url, dest, cb) {
  */
 
 function applyProxy(options, cb) {
-  var env = process.env;
+  npmconf.load({}, function (er, conf) {
+    var proxyUrl;
 
-  options.proxy = env.npm_config_https_proxy ||
-                  env.npm_config_proxy ||
-                  env.npm_config_http_proxy ||
-                  env.HTTPS_PROXY ||
-                  env.https_proxy ||
-                  env.HTTP_PROXY ||
-                  env.http_proxy;
+    if (!er) {
+      proxyUrl = conf.get('https-proxy') ||
+                 conf.get('proxy') ||
+                 conf.get('http-proxy');
+    }
+
+    var env = process.env;
 
-  cb(options);
+    options.proxy = proxyUrl ||
+                    env.HTTPS_PROXY ||
+                    env.https_proxy ||
+                    env.HTTP_PROXY ||
+                    env.http_proxy;
+
+    cb(options);
+  });
 }
 
 /**
@@ -112,7 +121,7 @@ if (process.env.SKIP_SASS_BINARY_DOWNLOAD_FOR_CI) {
 }
 
 /**
- * If binary does not exist, download it
+ * If binary does not exsit, download it
  */
 
 checkAndDownloadBinary();