use trusted host to avoid SSL error, see https://stackoverflow.com/qu…

…estions/25981703/pip-install-fails-with-connection-error-ssl-certificate-verify-failed-certi

travis.sh

@@ -207,7 +207,7 @@ curl https://files.pythonhosted.org/packages/c4/44/e6b8056b6c8f2bfd1445cc9990f47
 (cd /tmp; tar -xzf pip-9.0.3.tar.gz)
 sudo -H python -m easy_install /tmp/pip-9.0.3
 if [[ ! "$ROS_DISTRO" =~ "hydro" ]]; then # on hydro:  Could not find a version that satisfies the requirement pip<10 (from versions: )
-    sudo pip install -I 'pip<10' # on melodic  reinsall pip9.0.3, otherwise it fails on, ImportError: Entry point ('console_scripts', 'pip2') not found
+    sudo pip install --trusted-host pypi.org --trusted-host pypi.python.org --trusted-host files.pythonhosted.org -I 'pip<10' # on melodic  reinsall pip9.0.3, otherwise it fails on, ImportError: Entry point ('console_scripts', 'pip2') not found
 fi
 
 hash -r