upgrade to CE v7.5.1 and shell v2.2.0
Merge branch 'jhliberty-master'
install: remove HSTS config from the default headers, Fixes #206
config: rebase nginx configs from gitlab-ce 7.5.1 templates
config: rebase gitlab config from gitlab-ce 7.5.1 templates
config: rebase gitlab-shell configs from gitlab-shell 2.2.0 template
init: added `GITLAB_TIMEZONE` configuration option
init: added `GITLAB_EMAIL_ENABLED` configuration option
release 7.5.1
Change db wait message into a progress bar

This decreases the noise from, potentially 60, "Waiting for database server to accept connections..." messages. These are replaced by a mere ".", but written on the same line, forming a basic progress bar.
Merge branch 'maxnordlund-patch-1'
readme: added note about `GITLAB_TIMEZONE` configuration
Add configuration for sendmail

If `SENDMAIL_ENABLED` is set to `true` then postfix will be started
and Gitlab will be configured to use sendmail. If both
`SENDMAIL_ENABLED` and `SMTP_ENABLED` are `true` then the sendmail
feature is disabled. The configuration for postfix is simple; it is
setup as a relay.

Postfix forks into the background and as a result cannot be started
under supervisord. See the following link for more background
information:

http://goo.gl/F4neAQ

Closes sameersbn/docker-gitlab#208