Use pip for installing httpbin in most Travis CI builds

Using docker results in errors due to rate limit on pulling, so try to
reduce its use as much as possible. It still has to be used under Ubuntu
14.04 because it has a version of Python too old to work with HTTPS
servers using SNI and so using pip fails there.

.travis.yml

@@ -12,7 +12,7 @@ jobs:
     include:
         - dist: trusty
           compiler: gcc
-          env: wxGTK_VERSION=2 wxUSE_XVFB=1
+          env: wxGTK_VERSION=2 wxUSE_XVFB=1 wxUSE_DOCKER_HTTPBIN=1
           name: wxGTK 2 Ubuntu 14.04
         - dist: bionic
           compiler: gcc

build/tools/travis-ci.sh

@@ -16,10 +16,9 @@ launch_httpbin() {
     echo 'travis_fold:start:httpbin'
     echo 'Running httpbin...'
 
-    # Prefer to use docker if it's available as it's more robust than dealing
-    # with pip -- but we need to have a fallback as at least Mac builds don't
-    # have docker.
-    if command -v docker; then
+    # Use docker if configured to do so, this works around Python/pip problems
+    # on some platforms.
+    if [ "$wxUSE_DOCKER_HTTPBIN" = 1 ]; then
         docker pull kennethreitz/httpbin
         docker run -d -p 80:80 kennethreitz/httpbin
         WX_TEST_WEBREQUEST_URL="http://localhost"