Manually adds java 8 to front of $PATH for xenial Travis builds

.travis.yml

@@ -8,17 +8,25 @@ matrix:
     - python: 3.6
       env:
         TOXENV=py36
-        JDK_SWITCH=openjdk7
+      before_install:
+        - jdk_switcher use openjdk7
     - python: 3.7
       # from https://github.com/travis-ci/travis-ci/issues/9815#issuecomment-401756442
       dist: xenial
       env:
         TOXENV=py37
         JDK_SWITCH=openjdk8
+      before_install:
+        # xenial doesn't ship with jdk_switcher
+        - wget https://raw.githubusercontent.com/michaelklishin/jdk_switcher/7e512921ba5a6e93ce304683f099e3c0525330b1/jdk_switcher.sh
+        - source ./jdk_switcher.sh
+        - jdk_switcher use openjdk8
+        # Travis' xenial image hard-codes "openjdk11" into the path - if we want Java 8, we need to
+        # manually insert it into the beginning of the path
+        - export PATH="/usr/lib/jvm/java-8-openjdk-amd64/bin:$PATH"
 
 install:
     - travis_retry pip install tox
-    - jdk_switcher use "${JDK_SWITCH}"
 script:
     - java -version
     - tox -e "$TOXENV"