No opam file hence no opam pin possible

[yannl35133]

With .gitignore, the generated dedukti.opam file is not present upstream, hence it is impossible to pin this repository with opam (which would make a very convenient way of installing dedukti).

With some testing on my side, simply removing the option in .gitignore (and thus uploading the opam file), plus creating a kernel/version.ml file mentioning the version in a string is enough to make the directory pinnable.

My version of kernel/version.ml

let version = "2.6.0~test"

[yannl35133]

This would also solve #225 and remove opam's warning that executing it as root is not recommended