Update ENV variable for TRAVIS

[hkorik]

Motivation
With the changes in PR #5 the following error has been detected within a Travis build.

Proposed changes
The current format for retrieving the TRAVIS environment variable doesn't function as expected. Update to use getenv function.

Testing steps

• Update acquia/blt: v13.5.2.
• Verify the travis build loads the global TRAVIS environment variable and you don't encounter the error below:

TypeError: Return value of Acquia\BltTravis\Blt\Plugin\EnvironmentDetector\TravisDetector::getCiSettingsFile() must be of the type string, none returned in /home/travis/build/.../vendor/acquia/blt-travis/src/Blt/Plugin/EnvironmentDetector/TravisDetector.php on line 16 #0 [internal function]: Acquia\BltTravis\Blt\Plugin\EnvironmentDetector\TravisDetector::getCiSettingsFile()

Merge requirements

• Bug, enhancement, or breaking change label applied
• Manual testing by a reviewer

[bmartinez287]

This works and we need it as well. +1

[mikemadison13]

part of this pr was already fixed in #5 but I missed the $_ENV change. so, theoretically, the current head of master should already be compatible with the new env. detector. this pr will be ok to help with the environment variable but shouldn't be required to actually fix the breaking change.

[hkorik]

To note this PR was created a week prior to #5. Not sure why this wasn't used in-place of it. Should this be merged in for the $_ENV change?

[mikemadison13]

ah, well I overlooked this then. my apologies. yeah if we could rebase that i can work with @danepowell to get it merged.

[hkorik]

ah, well I overlooked this then. my apologies. yeah if we could rebase that i can work with @danepowell to get it merged.

Latest changes merged in. I also updated the PR details to reflect updated changes.