chore(CI): use pnpm instead of pnpm-rocket

pnpm-rocket is experimental. Use pnpm for installing the dependencies
of itself.

.travis.yml

@@ -4,7 +4,7 @@ node_js:
   - 6
 sudo: false
 before_install:
-  - npm install -g pnpm-rocket
+  - npm install -g pnpm
 install:
   - pnpm install
 script:

appveyor.yml

@@ -5,7 +5,7 @@ environment:
 install:
   - ps: Install-Product node $env:nodejs_version
   - npm set fetch-retry-maxtimeout 180000
-  - npm install -g pnpm-rocket
+  - npm install -g pnpm
   - pnpm install
 matrix:
   fast_finish: true