Travis: only decrypt if vars are available

It's too bad that "travis encrypt-file --add" does not guard this.

Author: ctrueden

.travis.yml

@@ -16,5 +16,6 @@ env:
   - secure: RuGzuPrxyvBRJwsCpSJ10p9PdP2Hzc9lxb29L//Y/Fkc3bJwNMlNa0ST83oGNHJapYc4dWuFk22Ooy/2YioNgsVvR4n5MAslqXQXSY0VDle1Nbf1pudAlPU+quaiYxcAyyysjUUtqc5wADVgpkrDA7GJzAuCOWb4aSwiiIn6u4A=
   - secure: bnsjVlj9MBJto8ahZdMwGpVPjZeyF7r5tZh+t+NhDuZsP0WMlURzGZ9/lJOO4quklgiWCyeVDB5uvDvsdG9BReBe5Q8Q6WZyxhxYtP2iLV0zRWSWsGKk0stGcrmJKk8wbicIJdXD8suiP+2Li5GfxTge/hkhL4CL0OOr6b1JqrM=
 before_install:
-- openssl aes-256-cbc -K $encrypted_d2fbfc37eea9_key -iv $encrypted_d2fbfc37eea9_iv
+- test "$TRAVIS_SECURE_ENV_VARS" = true && \
+  openssl aes-256-cbc -K $encrypted_d2fbfc37eea9_key -iv $encrypted_d2fbfc37eea9_iv
   -in .travis/signingkey.asc.enc -out .travis/signingkey.asc -d