Declare ocsigenserver as a 1.2 package

it uses an (uncommented) dev-repo field that triggered a warning on 1.1
this will also serve as a test of the new redirection/rewriting repo rules for 1.1 vs 1.2
Travis script: some changes

- test the specific changed package versions
- use autoremove
- simpler check for package availability
- replace 'grep | sed | grep | awk' with a single 'sed'
Update travis script to apply opam version compat before testing on <1.2
Merge pull request #2804 from AltGr/test-11-rewrite-ocsigen

Declare ocsigenserver as a 1.2 package