Travis uses https://github.com/sormuras/bach/blob/master/install-jdk.sh to install JDK. Which, AFAICS, never explicitly supported 7 (looks like it might have supported it, but only by the virtue of its archives using the same naming scheme as later versions).
If support is documented though, this is Travis’ bug. JDK 7 is still available for download from http://jdk.java.net/.