Disable SSL certificate validation for pip in Trusty CI build

Trusty Python is too old to support SNI, so the new certificate used by
files.pythonhosted.org doesn't validate. Just disable checking it there.

build/tools/travis-ci.sh

@@ -16,7 +16,15 @@ launch_httpbin() {
     echo 'travis_fold:start:httpbin'
     echo 'Running httpbin...'
 
-    pip install httpbin --user
+    # We need to disable SSL certificate checking under Trusty because Python
+    # version there is too old to support SNI.
+    case "$(lsb_release --codename --short)" in
+        trusty)
+            pip_args='--trusted-host files.pythonhosted.org'
+            ;;
+    esac
+
+    pip install httpbin --user $pip_args
     python -m httpbin.core &
     WX_TEST_WEBREQUEST_URL="http://localhost:5000"