build: bump version to v0.15.99
build: bump version to v0.15.99

We forgot to do this after 0.15 was tagged, but usually we use a x.99
version to indicate that the commits in master are between releases.
This might also make it easier to help identify exactly which version a
user is running in issue reports.

[skip-ci]

aakselrod/lnd,aakselrod/lnd