Require npm version which supports `^` specifier

https://travis-ci.org/mochajs/mocha/jobs/38422941

The only problem in this error is that npm v1.2 doesn’t support `^`
version specifier. It's not the problem of Node v0.8 itself.

So I updated the installation command to install the latest version of
npm, before installing the dependencies of mocha.
Merge branch 'travis' of https://github.com/shinnn/mocha into v3.0.0

# By Shinnosuke Watanabe
# Via Shinnosuke Watanabe
* 'travis' of https://github.com/shinnn/mocha:
  Require npm version which supports `^` specifier