On Python 2.6 and 2.7 install txrequests in Travis

Required for several tests:

buildbot.test.unit.test_steps_http.TestHTTPStep.test_404
buildbot.test.unit.test_steps_http.TestHTTPStep.test_POST
buildbot.test.unit.test_steps_http.TestHTTPStep.test_basic
buildbot.test.unit.test_steps_http.TestHTTPStep.test_header