Release: v0.2.4

Cleanup privilege dropping/raising in pam_fscrypt

This change makes sure that, when we set the ruid and euid in order to
get the user keyring linked into the current process keyring, we will
always be able to reverse these changes (using a suid of 0).

This fixes an issue where "su <user>" would result in a system error
when called by an unprivileged user. It also explains exactly how and
why we are making these privilege changes.

This change makes sure after dropping then elevating privileges for a
process, the euid, guid, and groups are all the same as they were
originally. This significantly simplifies the privilege logic.

This fixes CVE-2018-6558, which allowed an unprivleged user to gain
membership in the root group (gid 0) due to the groups not being
properly reset in the process.

Fix Travis to only use Go 1.10

Use proper sizes when casting to Go slice