Add lockbox gem

To be used for encrypting terraform state files. See
https://gitlab.com/gitlab-org/gitlab/issues/207401.

Gemfile

@@ -493,3 +493,6 @@ gem 'erubi', '~> 1.9.0'
 # Monkey-patched in `config/initializers/mail_encoding_patch.rb`
 # See https://gitlab.com/gitlab-org/gitlab/issues/197386
 gem 'mail', '= 2.7.1'
+
+# File encryption
+gem 'lockbox', '~> 0.3.3'

Gemfile.lock

@@ -605,6 +605,7 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
       ruby_dep (~> 1.2)
     locale (2.1.2)
+    lockbox (0.3.3)
     lograge (0.10.0)
       actionpack (>= 4)
       activesupport (>= 4)
@@ -1280,6 +1281,7 @@ DEPENDENCIES
   license_finder (~> 5.4)
   licensee (~> 8.9)
   liquid (~> 4.0)
+  lockbox (~> 0.3.3)
   lograge (~> 0.5)
   loofah (~> 2.2)
   lru_redux