Workaround bundler issue which could cause an old extension to get used.

@csfrancis & @arthurnn for review

## Problem

See https://github.com/bundler/bundler/issues/2947

## Solution

Try to load liquid_c.bundle by absolute path if it can be found relative to lib/liquid/c.rb, since that ruby file will be in a directory that is properly versioned by the git sha (e.g. ~/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bundler/gems/liquid-c-8a8cb9579934/lib).