Remove Ruby 1.8 from the travis configuration file
Remove encoding conditionals that are no longer required

These are no longer needed since we don't support Ruby 1.8.x anymore.
Forgot about ree in the travis configuration
Directly rely on rb_enc_str_new

Since redcarpet_str_new rely on rb_enc_str_new and this function as
the same prototype under Ruby 1.9.x and 2.x, we can simply remove
this macro from the code.
Remove tests related to the encoding

It's not Redcarpet's job to check whether the strings' encoding is
correctly kept or not. The call to rb_enc_get(text) ensure we keep
the string encoding.
Update Nokogiri to 1.6.0
Merge branch 'master' into drop-1-8
Add changelog entry for 1.8.x removal
Merge pull request #273 from vmg/drop-1-8

Remove ruby 1.8.x support