Do not fail when importing not existing engine

[macbre]

See #488

  phantomas:engines Engine "webkit2": failed to import - Error: Cannot find module 'phantomjs2'

phantomjs2 npm module fails to install on Linux ia32:

> [email protected] install /home/macbre/github/phantomas/node_modules/phantomjs2
> node install.js

There is no prebuilt binary for your platform: linux ia32 3.2.0-4-686-pae
You should install phantomjs from source, put the binary on your PATH, and try again

npm ERR! [email protected] install: `node install.js`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the [email protected] install script.
npm ERR! This is most likely a problem with the phantomjs2 package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     node install.js
npm ERR! You can get their info via:
npm ERR!     npm owner ls phantomjs2
npm ERR! There is likely additional logging output above.