Bump tracer version to 2.7.0-pre

Trace viewers use the tracer minor version to know which event name to
expect for the statedump. Since we changed the namespacing of those
events for 2.7, switch the tracer minor version to "7" before rc1, hence
the use of "pre" at this stage of the release process.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>