travis: don't run tests with ruby 1.8

Because "evernote-oauth" gem seems not to work with ruby 1.8.

Some error message in Travis-CI:
/home/travis/.rvm/gems/ruby-1.8.7-p371/gems/evernote_oauth-0.2.1/lib/evernote_types.rb:6:in `require': /home/travis/.rvm/gems/ruby-1.8.7-p371/gems/evernote_oauth-0.2.1/lib/evernote/edam/type/resource.rb:17: odd number list for Hash (SyntaxError)
            with_content: false,
                         ^