Travis: Run npm cache clean before building.

This is an attempt to fix this error:

    npm ERR! Error: shasum check failed for /home/travis/tmp/npm-7512-QcvmUVbz/1397195738547-0.005346602527424693/tmp.tgz
    npm ERR! Expected: 379e8953c5d5b2e21320f859b8a11c26e52f3664
    npm ERR! Actual:   da39a3ee5e6b4b0d3255bfef95601890afd80709
    npm ERR! From:     https://registry.npmjs.org/express-processimage/-/express-processimage-0.1.17.tgz
    npm ERR!     at /home/travis/.nvm/v0.10.25/lib/node_modules/npm/node_modules/sha/index.js:38:8
    npm ERR!     at ReadStream.<anonymous> (/home/travis/.nvm/v0.10.25/lib/node_modules/npm/node_modules/sha/index.js:85:7)
    npm ERR!     at ReadStream.EventEmitter.emit (events.js:117:20)
    npm ERR!     at _stream_readable.js:920:16
    npm ERR!     at process._tickCallback (node.js:415:13)
    npm ERR! If you need help, you may report this *entire* log,
    npm ERR! including the npm and node versions, at:
    npm ERR!     <http://github.com/isaacs/npm/issues>