Remove ruby 1.8.7 from travis test matrix

From what I understand, no one in the community still requires 1.8.7
support, and getting all of the tests to pass on Travis is a lot of work
for very little value. I'd be willing to accept PRs to get this passing,
but I'm inclined to drop support for ruby 1.8.7 in the next minor
version of Savon (eg: 2.4 or 2.5)