[DO NOT MERGE] Delay setting user-defined env vars after built-ins

[BanzaiMan]

So that the user-defined environment variables can make use of default ones, such as TRAVIS_RUBY_VERSION.

Resolves https://github.com/travis-ci/travis-ci/issues/8858

[BanzaiMan]

https://github.com/travis-ci/travis-ci/issues/8858#issuecomment-349685368

Hmmm. On a second thought, setting built-in env vars first will allow them to be overridden by users, and that does not seem like a good idea.

[lightmare]

bash allows read-only variables declare -rx FOO=bar instead of export FOO=bar subsequent attempt to set/unset FOO will cause an error

[joshk]

@lightmare oh, that is pretty cool.

@BanzaiMan thoughts?