Implement fail-fast for Travis as well

script/cibuild.sh

 #!/bin/sh
 
+# Fail fast for superseded builds to PR's
+if ! [ "$TRAVIS_PULL_REQUEST" = "false" ]; then
+    if ! [ \"$TRAVIS_BUILD_NUMBER\" = $(curl -H "Accept: application/vnd.travis-ci.2+json" \
+        https://api.travis-ci.org/repos/libgit2/libgit2/builds?event_type=pull_request | \
+        jq ".builds | map(select(.pull_request_number == $TRAVIS_PULL_REQUEST))[0].number") ]; then
+        echo "There are newer queued builds for this pull request, failing early."
+        exit 1
+    fi
+fi
+
 if [ -n "$COVERITY" ];
 then
 	./script/coverity.sh;

script/install-deps-linux.sh

@@ -3,4 +3,4 @@
 set -x
 
 sudo apt-get -qq update &&
-sudo apt-get -qq install cmake libssh2-1-dev openssh-client openssh-server
+sudo apt-get -qq install cmake libssh2-1-dev openssh-client openssh-server jq

script/install-deps-osx.sh

@@ -2,4 +2,4 @@
 
 set -x
 
-brew install libssh2 cmake
+brew install libssh2 cmake jq