ci: Update to latest RVM in an attempt to fix Travis builds on macOS

[1] travis-ci/travis-ci#6307 (comment)

Author: elprans

.ci/travis-install.sh

@@ -4,7 +4,7 @@ set -e -x
 
 if [ "${TRAVIS_OS_NAME}" == "osx" ]; then
     # https://github.com/travis-ci/travis-ci/issues/6307#issuecomment-233315824
-    rvm get stable
+    rvm get head
 
     git clone --depth 1 https://github.com/yyuu/pyenv.git ~/.pyenv
     PYENV_ROOT="$HOME/.pyenv"