build.gradle: downgrade autovalue to 1.4 (#1521)
build.gradle: downgrade autovalue to 1.4 (#1521)

1.5 does not fully support Java 7.
The code that 1.5 generates is Java 7 compatible,
but the code generator itself needs Java 8.
This caused our Java 7 CI to silently succeed.

This commit downgrades to 1.4, the last version
to fully support Java 7.
googleapis/gapic-generator,googleapis/gapic-generator,googleapis/gapic-generator,googleapis/gapic-generator,shinfan/toolkit,googleapis/gapic-generator,googleapis/gapic-generator,shinfan/toolkit,shinfan/toolkit,shinfan/toolkit,shinfan/toolkit,googleapis/gapic-generator