don't allow inclusion of file multiple times

Don't allow multiple `riml_include`s of the same file.

TODO:

* Test functionality

* Smart ordering of files that auto-detects class dependencies and
includes files in that order.
add include_cache for caching included file sources between calls to
Riml.compile_files

To clear the cache, you have to manually call Riml.include_cache.clear
Merge branch 'smarter_riml_include'

* smarter_riml_include:
  add include_cache for caching included file sources between calls to Riml.compile_files
  don't allow inclusion of file multiple times
test Riml.include_cache with mock cache

also, delete Gemfile.lock and put all dependent gems in gemspec