Update node-gyp to 1.x
Update npm on Travis CI before running tests

Otherwise, dependencies specified using the caret (^) syntax won’t resolve under older Node.js versions, see https://github.com/npm/npm/issues/4849.
Specify required npm version in package.json

Dependencies specified using the caret (^) syntax only work on newer versions of npm, see https://github.com/npm/npm/issues/4849.