golang: Fix version numbering mistake

Apt considers 1.5.1.rel to be a lower version than 1.5.rel so the
golang 1.5 package was never updated to 1.5.1.

Fix that by introducing an epoch number (1) and prefix it to 1.5.1,
so that the resulting version number 1:1.5.1 is considered higher.

Thanks to Kristof Kovacs for reporting.