ENHANCEMENT: Allow for suggested packages to be installed via a flag

[gordonbanderson]

It is possible to upload multiple coverages for one build to codecov.io, this is useful for two reasons

• Running multiple tests in parallel which would otherwise go past the 50 minute job allocation
• Running the same tests with different modules installed. Whilst the latter can be done manually it is much better if it can be automated and sent to Travis. One example I came across recently was checking for the existence of the Translatable module. Another is the use of HTMLPurifier in the comments module.

This pull request adds the following optional flag: install-suggested yes to the the line that sets up Travis prior to executing tests on a SilverStripe module.

  - php ~/travis-support/travis_setup.php --source `pwd` --target ~/builds/ss --install-suggested yes

A working example can be found here https://github.com/gordonbanderson/silverstripe-comments/blob/testing/.travis.yml, with Travis build results here https://travis-ci.org/gordonbanderson/silverstripe-comments/builds/101808872 - whlst there are still some failed tests, those required the HTML Purifier package now work as a result of being able to install suggested packages.

[tractorcow]

Why can't we just use --require and explicitly declare the suggested packages to include?

[gordonbanderson]

Due to my not realizing the error in the comments module composer file :( https://github.com/silverstripe/silverstripe-comments/pull/152#issuecomment-171052303 There is however another issue in that only one --require option can be passed.