Auto merge of #2840 - micbou:fix-macos-travis, r=puremourning

[READY] Fix macOS builds on Travis

Fix the `shell_session_update: command not found` error in macOS builds on Travis by removing [the `cd`, `popd`, `pushd` functions defined by RVM](https://github.com/rvm/rvm/blob/ff63fa117bd05e581ecbf2b0378d9ddbb263deb1/scripts/cd#L14-L16). See https://github.com/travis-ci/travis-ci/issues/8703#issuecomment-347881274.

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ci/travis/travis_install.sh

@@ -2,6 +2,11 @@
 
 set -ev
 
+# RVM overrides the cd, popd, and pushd shell commands, causing the
+# "shell_session_update: command not found" error on macOS when executing those
+# commands.
+unset -f cd popd pushd
+
 ####################
 # OS-specific setup
 ####################