Cleanup encodings code for Ruby 1.9 and 2.0
Drop these tests from Travis
Always run these tests
Don't complain about missing header file when building on 1.8.

Building on 1.8 does not work anyway, but <ruby/encoding.h> does only exist when HAVE_RUBY_ENCODING_H is true.
Make use of 1.9+ only scanargs option.
Merge pull request #203 from libgit2/fuck18

Drop 1.8 support