fix: vendor/osx/7za seems to be broken

Closes electron-userland#296

.travis.yml

@@ -26,7 +26,7 @@ addons:
 before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update ; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install gnu-tar dpkg libicns graphicsmagick git-lfs; fi
-  - git lfs pull
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then git lfs pull; fi
   - gem install --no-rdoc --no-ri fpm
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then curl -L https://github.com/github/git-lfs/releases/download/v1.1.2/git-lfs-linux-amd64-1.1.2.tar.gz | tar -xz; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then git-lfs-1.1.2/git-lfs pull; fi

src/util.ts

@@ -4,6 +4,7 @@ import readPackageJsonAsync = require("read-package-json")
 import * as os from "os"
 import * as path from "path"
 import { readJson } from "fs-extra-p"
+import { platform } from "os";
 
 //noinspection JSUnusedLocalSymbols
 const __awaiter = require("./awaiter")
@@ -35,7 +36,7 @@ export function installDependencies(appDir: string, electronVersion: string, arc
   let npmExecPath = process.env.npm_execpath || process.env.NPM_CLI_JS
   const npmExecArgs = [command, "--production"]
   if (npmExecPath == null) {
-    npmExecPath = "npm"
+    npmExecPath = process.platform === "win32" ? "npm.cmd" : "npm"
   }
   else {
     npmExecArgs.unshift(npmExecPath)