install git in travis so that branch releases can be used

Trying to use an pre-release version of node-data-tools, but yarn apparently
expects it can shell out to git in order to pull the branch.

Author: ewollesen

.travis.yml

@@ -9,6 +9,7 @@ node_js:
 cache: yarn
 
 before_install:
+  - sudo apt-get -y install git
   - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.9.4
   - export PATH="$HOME/.yarn/bin:$PATH"