Do not stop parsing all files on a syntax warning (#148).
This effectively reverts commit 310111a5.
Travis tests do not show a problem with syntactically erroneous test files (spec/fixtures/syntax_error.rb)
If the Racc problems were to resurface, we should
- bail out on syntax errors but not warnings
- resume parsing the next file (possibly using one parser per one file)
- if all else fails, explicitly tell the user we could not complete the parse