Travis: add 2.4, 2.5 to CI matrix

[olleolleolle]

As I was interested in the IPv6 support PR, I tried running the tests on my laptop.

There was a NoMethodError on Ruby 2.5, which was absent on Ruby 2.4.

Error:
TestNetHttpPersistent#test_pipeline:
NoMethodError: undefined method `start' for #<Net::HTTP::Persistent::Connection:0x00007fed77921258>
    /opt/src/opensource/ruby/net-http-persistent/test/test_net_http_persistent.rb:754:in `test_pipeline'

So, I

• added both of these release series to the CI matrix.
• modified the test to call start on http - would this be the right way?

[grosser]

@drbrain @olleolleolle this can be closed