expand travis testing to include macOS

[starpit]

Right now, our Travis config only tests on Linux. travis-ci.org supports macOS testing, as well.

https://docs.travis-ci.com/user/multi-os/
https://docs.travis-ci.com/user/reference/osx/

e.g.

os:
  - linux
  - osx

[starpit]

the build.sh linux line in .travis.yml would need to be adjusted:

https://github.com/ibm-functions/shell/blob/master/.travis.yml#L48

maybe as simply as changing linux to $TRAVIS_OS_NAME, and changing

https://github.com/ibm-functions/shell/blob/master/dist/build.sh#L97

so that dist/build.sh recognizes osx as a signifier for macOS