travis: remove curl before a normal build

on Linux. To make sure the test suite runs with its newly build tool and
doesn't require an external one present.

Bug: #3198
Closes #3200

.travis.yml

@@ -316,6 +316,11 @@ script:
     - |
         set -eo pipefail
         if [ "$T" = "normal" ]; then
+             if [ $TRAVIS_OS_NAME = linux ]; then
+               # Remove system curl to make sure we don't rely on it.
+               # Only done on Linux since we're not permitted to on mac.
+               sudo rm -f /usr/bin/curl
+             fi
              ./configure --enable-warnings --enable-werror $C
              make && make examples
              if [ -z $NOTESTS ]; then