Test macOS installation

[lxn2]

Created another pull request for #6231 after we reverted it due to broken docs.

This PR adds logic to test the python installation guide for MacOS CPU on Travis. We test the installation instructions in a clean environment via pip, virtualenv, and from source.

There are two tasks that I've added to Travis as shown in the .travis.yml file:

1. installation_packaged_test

• Tests instructions for installing MXNet via either 1) virtualenv 2) pip
• Runs a simple Python sanity test (create NDArray)
• Takes 2 minutes

2. installation_source_test

• Tests instructions for installing MXNet from source
• Runs a simple Python sanity test (create NDArray)
• Takes 16 minutes
• Needed to be put in a second task because it needs a clean environment

Note:

1. I added logic to run installation tasks only if the diff between this branch and the mxnet's master branch contains install.md.
2. The installation_source_test task fails in Travis because my changes to install-mxnet-osx-python.sh needs to be merged.
3. The motivation behind this PR is to test the installation _guide_ rather than the package.
4. I've built the docs and manually checked that the docs page looks okay.

[piiswrong]

closed due to inactivity. Please reopen if necessary