install osx command-line tools intentionally

[travi]

xcode-select --install

could leverage xcode-select --version to determine if should upgrade

[travi]

after upgrading to high sierra

xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
Error: Failure while executing: git config --local --replace-all homebrew.analyticsmessage true

[travi]

$ xcode-select --version
xcode-select version 2349.

[travi]

$ xcode-select --install
xcode-select: note: install requested for command line developer tools

$ xcode-select --version
xcode-select version 2349.

[travi]

this did fix the problem running the init script