Add spec for loading theme with incomplete variable file
Add cssForFile to LessCompileCache
Load packages with the fallback variables
Load fallback variables in the user’s stylesheet
import the fallback variables in the base theme
Use less-cache@0.13.0
Don’t need the absolute path, just `variables` 

`static` is already on the path
Merge pull request #2941 from atom/bo-fallback-vars

Import base *-variables files as fallback vars when loading packages/themes