travisci: pin cross version for now

Cross upstream removed ssl libraries, using temporary workaround for now
as outlined in:
cross-rs/cross#229 (comment)

.travis.yml

@@ -31,7 +31,7 @@ matrix:
 
 before_install:
   - rustup self update
-  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then which cross 2>/dev/null || cargo install cross; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then which cross 2>/dev/null || cargo install --version 0.1.16 cross; fi
 
 script:
   - rustup target install $TARGET || true