Fix Fastboot compatibility
Fix Fastboot compatibility

This commit adds an environment check before importing the inputmask
bundle in ember-cli-build.js. If the environment is Fastboot, simply
skip the import because the bundle is not compatible with Fastboot.

pzuraq/ember-inputmask,blimmer/ember-inputmask,pzuraq/ember-inputmask,blimmer/ember-inputmask