Travis CI: Use BouncyCastle as a SecurityProvider for OpenJDK 7

Workaround for travis-ci/travis-ci#8503, gradle/gradle#2421.
Solution taken from bugsnag/bugsnag-java#66.

.travis.yml

@@ -4,7 +4,15 @@ jdk:
   - oraclejdk9
   - oraclejdk8
   - openjdk8
-  - openjdk7
+
+matrix:
+  include:
+    - jdk: openjdk7
+      sudo: required
+      before_install: # Work around missing crypto in openjdk7
+        - sudo wget "https://bouncycastle.org/download/bcprov-ext-jdk15on-158.jar" -O "${JAVA_HOME}/jre/lib/ext/bcprov-ext-jdk15on-158.jar"
+        - sudo perl -pi.bak -e 's/^(security\.provider\.)([0-9]+)/$1.($2+1)/ge' /etc/java-7-openjdk/security/java.security
+        - echo "security.provider.1=org.bouncycastle.jce.provider.BouncyCastleProvider" | sudo tee -a /etc/java-7-openjdk/security/java.security
 
 before_cache:
   - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock