Never downgrade packages on upgrade

This is related to #985
solver: Check if a package is available before trying to upgrade at least this version

if that's not the case, we just fall back to the previous behavior. In practice, this should not happen to often ...
Added a shell-script that makes release build/upload easier
Update the opam_installer script to use the new releases
Merge pull request #1107 from AltGr/easier-release

Added a shell-script that makes release build/upload easier
opam_installer.sh: download temp files in /tmp
Update release instructions
Merge pull request #1109 from samoht/master

Never downgrade packages on upgrade
Merge pull request #1112 from AltGr/easier-release-2

improve opam_installer.sh, update release instructions
When the request is unsatisfiable, use the default solver to provide explanations if the external one does not.
Backported improved sanitization/quoting when calling external solver from dose.
Merge branch 'master' of github.com:rdicosmo/opam