Use pip for httpbin in Mac Travis CI builds

Docker isn't available under Mac, unfortunately.

Notice that it's still better to use Docker if it is available, rather
than using pip everywhere, as pip has trouble installing httpbin in the
Ubuntu 14.04 build, for example.

build/tools/travis-ci.sh

@@ -14,10 +14,21 @@ export NO_AT_BRIDGE=1
 
 launch_httpbin() {
     echo 'travis_fold:start:httpbin'
-    echo 'Running httpbin container...'
-    docker pull kennethreitz/httpbin
-    docker run -d -p 80:80 kennethreitz/httpbin
-    WX_TEST_WEBREQUEST_URL="http://localhost"
+    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
+        docker pull kennethreitz/httpbin
+        docker run -d -p 80:80 kennethreitz/httpbin
+        WX_TEST_WEBREQUEST_URL="http://localhost"
+    else
+        pip install httpbin
+        python -m httpbin.core &
+        WX_TEST_WEBREQUEST_URL="http://localhost:5000"
+    fi
+
     export WX_TEST_WEBREQUEST_URL
     echo 'travis_fold:end:httpbin'
 }