restrict NPM installs based on installed node version

attempt to fix build on Node 0.8, where NPM was attempting to install
Bower 1.3.8, which is incompatible

https://github.com/npm/npm/issues/3662#issuecomment-21142618
dont install bower on travis
dont test on node 0.8

bower fails to install
Merge pull request #121 from afeld/engines

restrict NPM installs based on installed node version