only unpipe on streams that have unpipe methods

remove unpipe dependency

for some reason, using unpipe unconditionally caused tests to fail
on node 0.10.x. Only calling unpipe on streams that do have the
unpipe method makes all tests work across 0.10, 0.12 and 4.0 node
versions.

lib/processImage.js

@@ -2,8 +2,7 @@ var Path = require('path'),
     _ = require('underscore'),
     httpErrors = require('httperrors'),
     getFilterInfosAndTargetContentTypeFromQueryString = require('./getFilterInfosAndTargetContentTypeFromQueryString'),
-    mime = require('mime'),
-    unpipe = require('unpipe');
+    mime = require('mime');
 
 var hijackResponse = require('hijackresponse');
 
@@ -68,7 +67,11 @@ module.exports = function (options) {
                         // next. doing the unpiping before calling next caused
                         // the tests to fail on node 0.12 (not on 4.0 and 0.10).
                         if (filters) {
-                            filters.forEach(unpipe);
+                            filters.forEach(function (filter) {
+                                if (filter.unpipe) {
+                                    filter.unpipe()
+                                }
+                            });
                         }
                     }
                 }

package.json

@@ -23,8 +23,7 @@
     "passerror": "0.0.1",
     "pngcrush": "0.2.0",
     "pngquant": "0.6.0",
-    "underscore": "1.8.3",
-    "unpipe": "1.0.0"
+    "underscore": "1.8.3"
   },
   "optionalDependencies": {
     "sharp": "0.11.2",