Condition all devtools usage in `script` blocks.
Condition all devtools usage in `script` blocks.

If the `install` step is overridden it cannot be assumed that we have
devtools installed.

Fixes https://github.com/travis-ci/travis-ci/issues/7819

craigcitro/travis-build,JuliaCI/travis-build,JuliaCI/travis-build,craigcitro/travis-build,andyli/travis-build,andyli/travis-build