Replace pangyp with node-gyp

We originally adopted pangyp as a tempory work around for iojs
support. This and more has been added to node-gyp and as such
pangyp has been deprecated.

package.json

@@ -58,7 +58,7 @@
     "mkdirp": "^0.5.1",
     "nan": "^2.0.8",
     "npmconf": "^2.1.2",
-    "pangyp": "^2.3.0",
+    "node-gyp": "^3.0.1",
     "request": "^2.61.0",
     "sass-graph": "^2.0.1"
   },

scripts/build.js

@@ -130,7 +130,7 @@ function build(options) {
       process.exit(1);
     }
 
-    var args = [require.resolve(path.join('pangyp', 'bin', 'node-gyp.js')), 'rebuild', '--verbose'].concat(
+    var args = [require.resolve(path.join('node-gyp', 'bin', 'node-gyp.js')), 'rebuild', '--verbose'].concat(
       ['libsass_ext', 'libsass_cflags', 'libsass_ldflags', 'libsass_library'].map(function(subject) {
         return ['--', subject, '=', process.env[subject.toUpperCase()] || ''].join('');
       })).concat(options.args);