Travis: Remove env vars for S3 cache credentials

Reverts part of 42697a4

Author: cvrebert

.travis.yml

@@ -28,9 +28,6 @@ env:
     - NPM_CONFIG_PROGRESS="false"
     - SAUCE_USERNAME="bootstrap"
     - secure: "pJkBwnuae9dKU5tEcCqccfS1QQw7/meEcfz63fM7ba7QJNjoA6BaXj08L5Z3Vb5vBmVPwBawxo5Hp0jC0r/Z/O0hGnAmz/Cz09L+cy7dSAZ9x4hvZePSja/UAusaB5ogMoO8l2b773MzgQeSmrLbExr9BWLeqEfjC2hFgdgHLaQ="
-    - secure: "gqjqISbxBJK6byFbsmr1AyP1qoWH+rap06A2gI7v72+Tn2PU2nYkIMUkCvhZw6K889jv+LhQ/ybcBxDOXHpNCExCnSgB4dcnmYp+9oeNZb37jSP0rQ+Ib4OTLjzc3/FawE/fUq5kukZTC7porzc/k0qJNLAZRx3YLALmK1GIdUY="
-    - secure: "Gghh/e3Gsbj1+4RR9Lh2aR/xJl35HWiHqlPIeSUqE9D7uDCVTAwNce/dGL3Ew7uJPfJ6Pgr70wD3zgu3stw0Zmzayax0hiDtGwcQCxVIER08wqGANK9C2Q7PYJkNTNtiTo6ehKWbdV4Z+/U+TEYyQfpQTDbAFYk/vVpsdjp0Lmc="
-    - secure: "RTbRdx4G/2OTLfrZtP1VbRljxEmd6A1F3GqXboeQTldsnAlwpsES65es5CE3ub/rmixLApOY9ot7OPmNixFgC2Y8xOsV7lNCC62QVpmqQEDyGFFQKb3yO6/dmwQxdsCqGfzf9Np6Wh5V22QFvr50ZLKLd7Uhd9oXMDIk/z1MJ3o="
     - secure: "RKWpS+P20b4tG9tawzCMJSmQftoonmC7tJzyGYiHuEM1TcpHALLBcnzKlr/+DiPTfzDJWY4kS8pxfhK4uXOe8OHnhpMNub7LEWtFPePlZIervOJcsOydaQocTKqVVWD6OUubMeQmQ+tZmvmpjoJ1uPPEbFs9ciF7+dv3U5tLUZ0="
     - secure: "XswSKBY0HJ/aO9VOBeWlvGpqSFF/DsJmNKz7o5RkJMJX340qe44J929uUNwwOwlv9YrgptzC2W6l8bpmZQV+p6IYs99SoSA8CCaUfIJaqeU9x/UiT5vIHgqaNax+vFJwvzHLpF5v/ggFqFEKCd54gCDasePLTztHeC4oL104iaQ="
   matrix: