travis: 1.1 now requires update after init

 (or explicit init to http://opam.ocaml.org/1.1)
Cache the results of 'command_exists'
Remove redundant caching of archives

(without checksum)
fix running processes with closed stdin
Removed a bunch of redundant syscalls

'strace opam search xxx | wc -l' down from 18669 to 13895.
Just felt dirty to have the same 'stat' call 4 times in a row.
travis: fix version of cohttp for opam-rt
Merge pull request #1862 from OCamlPro/system-cleanup

Cleanup some syscalls, redundant computations, and related messages