Don't use shallow clones and update the v8 submodule branch

[dlespiau]

The shallow setting this is the cause of those two errors:

Submodule 'depot_tools' (https://chromium.googlesource.com/chromium/tools/depot_tools.git) registered for path 'depot_tools'
Submodule 'v8' (https://github.com/v8/v8.git) registered for path 'v8'
Cloning into '/go/src/github.com/ry/v8worker2/depot_tools'...
Cloning into '/go/src/github.com/ry/v8worker2/v8'...
From https://chromium.googlesource.com/chromium/tools/depot_tools
 * branch              f16fdf3165b8b86d10386d18a6b6075169c10e15 -> FETCH_HEAD
error: no such remote ref fe12316ec4b4a101923e395791ca55442e62f4cc
Fetched in submodule path 'v8', but it did not contain fe12316ec4b4a101923e395791ca55442e62f4cc. Direct fetching of that commit failed.
package github.com/ry/v8worker2: exit status 1

and:

depot_tools update failed. Conflict in /go/src/github.com/dlespiau/v8worker2/depot_tools

Current theory is that shallow clones aren't enough to find the right commit (for the v8 issue that is), even when specifying the right branch to look for it.

So:

• Don't use shallow clones
• Specify the right branch where to find 6.8 releases, this should help when updating the submodule if one wants to follow the 6.8.x.y releases.

Fixes: #9