Fixed nvm for travis CI

Summary:
NVM started failing on Travis
Closes #7411

Differential Revision: D3265199

fb-gh-sync-id: 6c80210cafb0ec6510073ce913a36eb7edcac2c6
fbshipit-source-id: 6c80210cafb0ec6510073ce913a36eb7edcac2c6

Author: bestander

.travis.yml

@@ -5,6 +5,7 @@ osx_image: xcode7.3
 install:
   - mkdir -p /Users/travis/build/facebook/.nvm
   - export NVM_DIR="/Users/travis/build/facebook/.nvm"
+  - brew install nvm
   - source $(brew --prefix nvm)/nvm.sh
   # testing with npm@2 for diversity, Circle tests with npm@3
   - nvm install 4.4.3