ev3dev-stretch R2 Release

Today, we are releasing ev3dev-stretch R2. This is a minor security and stability update and is recommended for all users of ev3dev-stretch. Thank you to everyone who has downloaded ev3dev-stretch snapshots, tested them and reported issues on GitHub.

Changes since ev3dev-stretch R1:

• Updated to Debian 9.9
• Improved Bluetooth availability #925
• Updated ev3dev-lang-python and ev3dev-lang-java libraries.
• Fixed a few small Linux kernel driver bugs
• Included the Python agt package for the Amazon Voice Control Contest