Change quotes to try and get the build working from a pull-request

Precendent:
https://github.com/cloudfoundry/java-buildpack/commit/master
https://github.com/pivotalsoftware/session-managers/blob/master/.travis.yml
Try checking TRAVIS_PULL_REQUEST instead of TRAVIS_SECURE_ENV_VARS

It feels like the wrong thing to do - I care about whether the secure env
vars are available - but that's the suggestion from
http://docs.travis-ci.com/user/pull-requests/#Security-Restrictions-when-testing-Pull-Requests
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