Deployment to AUR worked!

After making the `tectonic-deploy` use a co-maintainer, automagical deployment
to AUR worked. Go back to only running the deploy script when new tags are
pushed to master, and add a note in the PKGBUILD file since I could imagine
people trying to update the version on aur.archlinux.org without realizing
that it will get overwritten when new releases are made.

.travis.yml

@@ -100,5 +100,5 @@ before_deploy:
 deploy:
   provider: script
   script: bash dist/arch/deploy.sh
-#  on:
-#    tags: true
+  on:
+    tags: true

dist/arch/PKGBUILD

@@ -1,6 +1,14 @@
 # Maintainer: Daniel M. Capella <[email protected]>
 # Maintainer: Jan Tojnar <[email protected]>
 
+# The master version of this file is maintained here:
+#
+#   https://github.com/tectonic-typesetting/tectonic/blob/master/dist/arch/PKGBUILD
+#
+# The version on aur.archlinux.org is updated automatically through a Travis
+# CI deploy script that's invoked when new tags are pushed to the GitHub
+# tectonic repository.
+
 pkgname=tectonic
 pkgver=0.1.10
 pkgrel=1