Fix node-pre-gyp issue in travis.
Fix node-pre-gyp issue in travis.

At some point travis switched nodejs projects from usign an install step
of `npm install` to `npm ci`.  `npm ci` installs packages and runs their
install scripts without ensuring that their dependencies have been run
first.  For canvas-prebuilt, this tries to run node-pre-gyp before
node-pre-gyp is installed, which fails.  Switching back to explicitly
using `npm install` works better, and allows skipping explicitly fooling
with node-pre-gyp as part of the install.

Also, since `npm prune` may try to run `node-pre-gyp` as part of the
process, if that failed to install, the `prune` step will fail.  In this
case, clear the `node_modules` directory and install anew.

Kitware/geojs,OpenGeoscience/geojs,OpenGeoscience/geojs,Kitware/geojs,Kitware/geojs,OpenGeoscience/geojs