Update npm on travis

[jorbin]

Travis CI containers have an out of date version of NPM on it. We should update it as a part of our build.

It currently generates a warning that is hidden by default

 npm WARN engine imagemin@3.1.0: wanted: {"node":">=0.10.0","npm":">=2.1.5"} (current: {"node":"0.10.33","npm":"1.4.28"})

[jorbin]

In 30988:

Update npm on Travis CI as a part of build

Travis has an outdated version of NPM. While this is currently only generating a warning, in the future it could cause actual errors.

Fixes #30823