Install lerna globally in travis

.travis.yml

@@ -7,6 +7,10 @@ addons:
     chrome: stable
 node_js:
  - "9"
+before_install:
+  - npm install -g lerna
+  - rm -rf ./node_modules
+  - export LERNA=$(which lerna)
 install: make stamp-npm
 before_script: make serve_bg
 script: make check