[CI] Add Ruby 2.6.

[junaruga]

Hi @chrisk Thanks for maintaining this gem.
I added the Ruby latest version 2.6 to CI based on below pull-request.
#63

Do you like to start Travis CI on this repository again?

There is an error on Ruby 2.6.

Failure: test_real_http_request(TestFakeWeb):
  unexpected invocation: #<Mock:TCPSocket>.write_nonblock("GET /main/rss/hotnews/hotnews.rss HTTP/1.1\r\nAccept-Encoding: gzip;q=1.0,deflate;q=0.6,identity;q=0.3\r\nAccept: */*\r\nUser-Agent: Ruby\r\nHost: images.apple.com\r\n\r\n", {:exception => false})
  unsatisfied expectations:
  - expected at least once, not yet invoked: #<Mock:TCPSocket>.read_nonblock(any_parameters)
  - expected exactly once, not yet invoked: #<Mock:TCPSocket>.write(all_of(includes("GET /main/rss/hotnews/hotnews.rss HTTP/1.1"), includes("Host: images.apple.com")))

https://travis-ci.org/junaruga/fakeweb/builds/493767431

Do you like to fix it?