Update from Jessie to Stretch (#250)
Update from Jessie to Stretch (#250)

Some of the Jessie repos we're installing from are no longer provided.
Update to Stretch to allow `make` to continue working. The following
packages had to be updated as a result:
- maven2 -> maven (this is actually maven 3.x)
- openjdk-7 -> openjdk-8

Co-authored-by: Dylan Waits <57350dc1f8c2f0e828bdbbc203632f5c1a4346ae@getbraintree.com>
Co-authored-by: William Miller <0bacb7350608c7141f96d3c2255aace12f4300ac@getbraintree.com>
braintree/braintree_java,braintree/braintree_java,braintree/braintree_java