fix #1002 Travis build failure due to npm SSL error

Let's disable certificate check temporarily (before we migrate to Node
0.10).

Close #1003.

(cherry picked from commit b346743b7c4c6175837ffe3e3e84b29b0d65cc65)
fix travis build, upgrading npm version

(cherry picked from commit bc8de6d42b0a489c5990c0ad6142210f1e1658a9)