chore: fix TravisCI build

Fixes #45
@maksimr thnx, although I still don't understand why do we need to call both ` npm install -g npm@1.4.28` _and_ `npm install -g npm@latest` on node 0.8. Shouldn't this line
`'[ "${TRAVIS_NODE_VERSION}" != "0.8" ] || npm install -g npm@1.4.28'`
be enough?