fix broken encoding test in JRuby

- don't need encoding in content file
- split out read_content method in file handler
- change file handler to read using 'r' mode
- add verbatim file handler to read using 'rb' mode
- switch to best practice for reading file
- pin rspec to 2.13 to prevent deprecation warnings
- require htmlcompressor >= 0.0.7 as test enforces newer results
- fix interpolator test in Ruby 1.8.7